Applies to: Exchange Server 2007 SP3, Exchange Server
2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007
Topic Last Modified: 2008-03-12
Microsoft Exchange Server 2007 Unified Messaging (UM) combines voice messaging, fax, and e-mail messaging into a single messaging infrastructure. Unified Messaging puts all e-mail, voice, and fax messages into one Exchange 2007 mailbox that can be accessed from a variety of devices. After Unified Messaging servers have been deployed on the network, users can access their messages by using Outlook Voice Access, from any telephone, from a mobile device, or from the computer of a user who is running Microsoft Windows XP.
Today, people in organizations frequently manage their voice and fax messages separately from their e-mail messages. Additionally, IT administrators frequently manage the voice mail or telephony networks and the e-mail systems or data networks as separate systems. In these situations, voice mail and e-mail are located in separate inboxes that are hosted on separate servers that are accessed through the desktop for e-mail and through the telephone for voice mail. Fax messages come to, and are sent from, physical stand-alone fax machines. Exchange 2007 Unified Messaging uses the Exchange 2007 store for all messages, including e-mail, voice, and fax messages.
Benefits of Exchange 2007
Unified Messaging
The Exchange 2007 Unified Messaging solution offers benefits for the end user and also for the IT administrator.
User Benefits
When you deploy Exchange 2007 Unified Messaging, your users will have access to their e-mail, voice mail, and fax messages from either Microsoft Office Outlook 2007 or the version of Outlook Web Access that is included with Exchange 2007. Additionally, users will be able to use the following features:
- Access to Exchange information To offer
a seamless voice mail experience for the user, UM-enabled users can
access a full set of voice mail features from Windows Mobile
powered devices, Outlook 2007, and
Outlook Web Access. These features include many voice
mail configuration options and the ability to play a voice message
from either the reading pane by using an integrated Windows Media
Player or from the message list by using their computer
speakers.
- Play on Phone The Play on Phone feature
lets UM-enabled users play voice messages over a telephone. If a
UM-enabled user sits in an office cubicle, is using a public
computer or a computer that is not enabled for multimedia, or is
listening to a voice message that is confidential, the user might
not want to or be able to listen to a voice message through their
computer speakers. Alternatively, they can play the voice message
by using any telephone, including home, office, or cellular
telephones.
- Voice mail form The
Outlook 2007 voice mail form resembles the default
e-mail form. It gives users an interface for performing actions
such as playing, stopping, or pausing voice messages, playing voice
messages on a telephone, and adding and editing notes.
The voice mail form includes the embedded Windows Media Player and an audio notes field. The embedded Player and notes field are displayed in either the preview pane when a user previews a voice message or in a separate window when they open the voice message. If a user is not enabled for Unified Messaging or Outlook 2007 has not been installed on the client computer, they view voice messages as e-mail attachments, and the voice mail form is not available.
- Fax
receiving Exchange 2007 Unified
Messaging enables voice messages to be delivered into a user's
Exchange 2007 mailbox and also lets users receive fax
messages in their mailbox. A fax message is sent to the user's
mailbox as an e-mail message that has an image file that has a .tif
extension attached. When a user receives such a message in their
mailbox, they can open the attached file by using a software
application that can open and view image files that have a .tif
extension.
- User configuration A user who is
enabled for Unified Messaging can configure several voice mail
options for Unified Messaging by using
Outlook Web Access. For example, the user can configure
telephone access numbers and the voice mail Play on Phone number,
and can reset a voice mail access PIN.
- Call answering Call answering includes
answering an incoming call on behalf of a user, playing their
personal greeting, recording a message, and submitting it for
delivery to their Inbox as an e-mail message.
- Outlook Voice Access There are two
Unified Messaging user interfaces available to UM-enabled users or
subscribers: the Telephone User Interface (TUI) and the Voice User
Interface (VUI). In Exchange 2007, these two interfaces
together are called Outlook Voice Access. A subscriber can use
Outlook Voice Access when they access the Unified Messaging system
from an external or internal telephone. They can use Outlook Voice
Access to access their Exchange 2007 mailbox, including
their personal e-mail, voice messages, and calendar
information. Users can listen to, reply to, create, and
forward unread e-mail messages by using the telephone.
- Subscriber access The subscriber access
feature enables dial-in access for your organization's users.
UM-enabled users or subscribers who dial in to the Unified
Messaging system can access their mailbox by using Outlook Voice
Access. By using a telephone, a subscriber or user can:
- Access voice mail.
- Listen, forward, or reply to e-mail messages.
- Listen to calendar information.
- Access or dial contacts who are stored in the global address
list or a personal contact list.
- Accept or cancel meeting requests.
- Set a voice mail Out-of-Office message.
- Set user security preferences and personal options.
- Access voice mail.
- Auto attendant An auto attendant is a
set of voice prompts that gives external or internal users access
to the Exchange 2007 Unified Messaging system. Users can
use the telephone keypad or speech inputs to move through the auto
attendant menu structure, place a call to a user, or locate a user
and then place a call to that user. An auto attendant gives the
administrator the ability to:
- Create a customized menu for external users.
- Define informational greetings, business hours greetings, and
non-business hours greetings.
- Define holiday schedules.
- Describe how to search the organization's directory.
- Describe how to connect to a user's extension so external
callers can call a user by specifying their extension.
- Describe how to search the organization's directory so external
callers can search the organization's directory and call a specific
user.
- Enable external users to call the operator.
- Create a customized menu for external users.
Benefits for Administrators
Deploying Exchange 2007 Unified Messaging offers the following benefits to administrators:
- A complete unified messaging
system Exchange 2007 Unified Messaging
offers a true unified messaging solution by using a single store,
transport, and directory infrastructure. The store is provided by
the Exchange 2007 Mailbox server role. The transport is
provided by the Exchange 2007 Hub Transport server
role. By locating all e-mail and voice mail messages in a
single store, you can manage this information from a single
management point. For example, all e-mail, voice mail, and fax
messages can be backed up by using a single administration
interface and toolset. This greatly reduces the overall cost of
administration by consolidating infrastructure and training.
- An Exchange 2007 deployment and administration
model By using the Exchange 2007 Unified
Messaging solution, you take advantage of the Exchange 2007
server design. You are not required to purchase a new server to run
the Unified Messaging server role. More important, you can reuse
your Exchange knowledge, including training and troubleshooting
methodology, and apply it to managing your voice mail and
fax messaging infrastructure.
- An Exchange 2007 security model The
Microsoft Exchange Unified Messaging service runs as an Exchange
server account. This means that you do not have to create or
manage a super user account for Exchange 2007 Unified
Messaging.
- Consolidation of voice mail
systems Currently, most voice messaging
systems require that all the voice messaging system components be
installed in every physical office location in an organization. In
this kind of arrangement, the voice messaging systems in those
branch offices are located outside the central office and must be
administered onsite. This distributed management scenario can
frequently result in increased administration costs and complexity.
Exchange 2007 Unified Messaging lets you manage your voice
mail system from a central location. To create a centralized
management system for Unified Messaging, you can place
all Unified Messaging servers in a datacenter or location, and then
deploy IP gateways in each of your branch offices that replace
the voice messaging system for each branch office. Deploying a
centralized voice messaging system in this manner can result
in a significant savings in hardware and administrative costs.
- Speech enabled auto attendants When
internal or external callers call in to the
Exchange 2007 Unified Messaging system, a series of voice
prompts assists them in moving through the menu system called an
auto attendant. The auto attendant enables the caller to connect to
a person in an organization or locate a person in the organization
so that they can place a call without assistance from a human
operator. With an auto attendant, callers hear voice prompts
instead of a human operator or receptionist.
You can create multiple auto attendants in Exchange 2007 Unified Messaging. Within the Active Directory directory service, each auto attendant is represented as an object. Configuration settings for an auto attendant are made on the Active Directory object and can include language settings, customized menus, and other menu navigation settings. You can also configure each UM auto attendant so that when a user calls in to an auto attendant, the caller can use either dual tone multi-frequency (DTMF) inputs (also known as touchtone inputs) or voice inputs (available in English) to move through the Unified Messaging auto attendant menu system.
Unified Messaging
Architecture
The Unified Messaging server role is one of several server roles that you can install and then configure on a computer that is running Exchange Server 2007. Unified Messaging (UM) is new to the Microsoft Exchange product line, and its introduction brings new concepts that may not be familiar to an Exchange administrator.
Unified Messaging combines voice messaging, fax, and e-mail messaging in the Exchange store. Exchange 2007 Unified Messaging integrates Microsoft Exchange with telephony networks and brings the Unified Messaging features to the core of Exchange. The following figure illustrates the relationship between an organization's telephony network components and the Exchange 2007 Unified Messaging system.
In the previous figure, the Exchange 2007 Unified Messaging solution provides access to telephony systems by using standard Voice over IP (VoIP) protocols. These protocols include Session Initiation Protocol (SIP), Realtime Transport Protocol (RTP), and the T.38 protocol. The IP gateways provide interoperability for legacy Private Branch eXchange (PBX) systems.
Configuring the Unified
Messaging Server
The Unified Messaging server object that is created in Active Directory is an essential part of the Unified Messaging system. During an installation of the Unified Messaging server role, a Unified Messaging computer object is created in the Computers container in Active Directory. The Unified Messaging Active Directory computer objects are the connection between your organization's telephony infrastructure and the Exchange 2007 Unified Messaging Active Directory networking environment. The Unified Messaging computer object that is created in Active Directory is a logical representation of a physical server on which the Unified Messaging server role is installed.
 Note: |
|---|
| For a new Unified Messaging computer object to be created during installation, the Exchange 2007 Unified Messaging server must be a member of a domain before the Unified Messaging server role is installed. |
After the computer object has been created, you can perform the necessary procedures to successfully deploy Unified Messaging on your network.
Server Operation
A Unified Messaging server will not process incoming calls unless the operational state is set to Enabled. By default, the operational status of the Unified Messaging server is set to Enabled after installation This enables the Unified Messaging server to process incoming and outgoing voice calls and incoming fax calls and route the messages to the intended recipients in your Exchange organization.
Although the operational status of the Unified Messaging server is set to Enabled after installation, the Unified Messaging server also maintains a logical status parameter that is used to control the operational status of the Unified Messaging server. The intention of the logical status variable is to let you stop call processing so that the Unified Messaging server can be taken offline in a controlled way.
The operational status of the Unified Messaging server can be controlled by the Enable and Disable commands in the Exchange Management Console and the Exchange Management Shell. There are three status modes for Unified Messaging servers:
- Enabled Process all incoming calls.
- Disable immediately Do not accept any
new calls and drop all existing calls.
- Disable after completing calls
Do not accept any new calls but process all
existing calls.
When the Unified Messaging server starts, it locates all IP gateways that are associated with the UM dial plans and are associated with the Unified Messaging server. To detect and identify any configuration changes on either UM dial plans or UM IP gateways, the Unified Messaging server will either register a change notification or re-check the configuration every 10 minutes.
If the UM IP gateway list changes, the Unified Messaging server will react accordingly and either start to use or stop using the appropriate IP gateways. After a Unified Messaging server is working as an associated member of a UM dial plan and is communicating with an IP gateway or a PBX, you can run a set of diagnostic operations to verify the correct operation and connectivity.
Configuring Unified Messaging
Users
When you create an Exchange 2007 recipient, you are given the option to create a mailbox or to connect to an existing mailbox. After the mailbox is created for the user or the user is connected to an existing mailbox, you must enable the mailbox so that the user can use Exchange 2007 Unified Messaging. After the user is enabled for UM, all e-mail, voice, and fax messages will be delivered to the user's mailbox. By using Outlook 2007, Outlook Web Access, a mobile device that is enabled for Exchange ActiveSync, or a regular or cellular telephone, the user can access their e-mail, voice and fax messages, and calendaring information.
User UM Properties
By default, a user who has an Exchange 2007 mailbox is not enabled for UM. You must create a mailbox for the user before the user can be enabled for UM. After the user is enabled for UM, you can manage, modify, and configure the UM properties for the user.
| Note: |
|---|
| To enable multiple UM users, use the Enable-UMMailbox cmdlet in the Exchange Management Shell. |
There are two locations in which UM properties are stored for a user: the Mailbox object and the user's Active Directory object. When you enable a user for Unified Messaging, you set the UM property on the user's Mailbox object. After the Mailbox property is set to Enabled for Unified Messaging, the user can use the Unified Messaging features found in Exchange 2007.
After a user is enabled for UM, the user's Unified Messaging properties are stored in the user properties and the user's mailbox. The user's Unified Messaging properties, such as the user's extension number, spoken name, and other properties for the user, are stored in the user's properties in Active Directory.
You can manage Unified Messaging properties for an Active Directory user on the mailbox of the Exchange 2007 Unified Messaging user by using the Exchange Management Shell or the Exchange Management Console.
The Relationship of the UM User to Other Active Directory Objects
When you enable a user for Unified Messaging, the user must be associated with or linked to an existing UM mailbox policy and you must provide the extension number for the user. You can associate a user with a UM mailbox policy by using the Enable-UMMailbox cmdlet or by selecting the UM mailbox policy when you create the user's Exchange mailbox.
A UM mailbox policy contains settings such as the dialing restrictions and PIN policies for a user. When a UM mailbox policy is created, the UM mailbox policy must be associated with only one UM dial plan. The UM dial plan is then associated with at least one Unified Messaging server. Any Unified Messaging server that is associated with the UM dial plan can provide Unified Messaging services for a UM-enabled user who uses the UM dial plan. Associating these Active Directory objects in this manner delivers the Unified Messaging services by using Active Directory. After the user is enabled for UM, the settings from a UM mailbox policy object are applied to the UM-enabled user.
Configuring Unified Messaging
Components
The UM Active Directory objects enable the integration of Exchange 2007 Unified Messaging into Active Directory and the existing telephony infrastructure. Active Directory acts as a container for all the UM objects that are created and their configuration settings. Each UM object within Exchange 2007 is necessary to support Unified Messaging in an Active Directory environment. Some UM Active Directory objects are created to logically represent a telephony hardware device such as an IP gateway, and other UM Active Directory objects are created to represent a telephony dial plan for an organization or to support a specific feature of Unified Messaging.
The following example and figure illustrate the relationships between the Unified Messaging objects that are found in Active Directory.
In the previous figure, the Unified Messaging servers represented as Server01, Server02, and Server03 are located in the datacenter for a company. These servers are also represented as Unified Messaging Server objects in Active Directory. Additionally, there are two UM dial plans, the Boston UM dial plan and the New York UM dial plan. There is a single PBX for each office. However, there are two IP gateways per PBX at each branch office for fault tolerance. Finally, Server01 and Server02 are members of the Boston UM dial plan, and Server01, Server02, and Server03 are all members of the New York UM dial plan.
When a call comes in to the PBX in the New York branch office and the call is forwarded to a Unified Messaging server, any of the available three Unified Messaging servers can answer the call and deliver the message to the user's mailbox because all the UM servers belong to the same dial plan. When a call is received in Boston for a user and the call is forwarded to a Unified Messaging server, either Server01 or Server02 can answer calls for the users in the Boston dial plan.
Exchange 2007 Unified Messaging provides voice messaging services that take advantage of your current Active Directory environment. There are several key Active Directory objects that you will must create and manage. These objects are outlined in the following sections.
UM Dial Plans
Unified Messaging dial plans are integral to the operation of Exchange 2007 Unified Messaging and are required to successfully deploy Unified Messaging on your network. A Unified Messaging dial plan is an Active Directory container object that logically represents one or more PBXs that share common user extension numbers. In practical terms, users' extensions that are attached to PBXs share a common numbering plan. Users can dial one another's telephone extensions without dialing a full telephone number. A UM dial plan is a logical representation of a telephony dial plan.
UM dial plans in Exchange 2007 Unified Messaging require that user telephone extension numbers be unique. UM dial plans are implemented in Unified Messaging to ensure that user telephone extensions are unique. In some telephony networks, multiple PBXs and multiple dial plans can exist. In these telephony networks, there could be two different users who have identical telephone extensions. UM dial plans resolve this issue. You can put the two users who have the same extension number into two separate UM dial plans. This makes the users' numbers unique for Unified Messaging.
The following figure illustrates how Unified Messaging dial plans can be used in an organization that has a single forest and multiple physical sites.
How Dial Plans Work
When you integrate a telephony network together with Unified Messaging, there must be an IP gateway that connects your telephony network with your IP-based network. Each IP gateway in your organization is represented by a UM IP gateway object in Active Directory. Exchange 2007 Unified Messaging requires that you create at least one UM dial plan and that the UM dial plan has a Unified Messaging server and a UM IP gateway associated with it. After you install the Unified Messaging server role on a computer that is running Exchange 2007, you must associate the Unified Messaging server with at least one UM dial plan before it will answer calls. You can also associate a single Unified Messaging server with multiple UM dial plans. After the Unified Messaging server is associated with a UM dial plan, you must create a UM IP gateway and associate it with the UM dial plan that was created.
 Important: |
|---|
| Each time that you create a UM dial plan by using the Exchange Management Console, a UM mailbox policy will also be created. The UM mailbox policy will be named <DialPlanName> Default Policy. |
If you create the first UM IP gateway and do not specify a UM dial plan at the time that you create it, a default UM hunt group is also created. Creating and associating these objects in Active Directory enables the Unified Messaging server to receive calls from the IP gateway and then process incoming calls for users who are associated with the UM dial plan. When a call comes in to the IP gateway, the IP gateway forwards the call to a Unified Messaging server and the Unified Messaging server tries to match the extension number of the user to the associated UM dial plan. The dial plan is identified from the pilot numbers to which the gateway presents the call.
UM IP Gateways
A UM IP Gateway object is a container object that logically represents a physical IP gateway hardware device, IP-PBX, or other SIP server that can interoperate with Exchange Unified Messaging. Before the IP gateway can be used to process Unified Messaging calls, it must be represented by an object in Active Directory.
Although there are many types and manufacturers of PBXs, IP gateways, and IP PBXs, there are basically two types of IP gateway component configurations:
- IP PBX A single device
- PBX (legacy) and an IP gateway Two
separate components
To support Exchange 2007 Unified Messaging, one or both types of IP device configurations are used when connecting a telephony network infrastructure to a data network infrastructure.
UM IP Gateway Objects
The UM IP Gateway object is an Active Directory container object that contains one or more Active Directory UM hunt groups and other UM IP gateway configuration settings. UM IP Gateway objects are created in Active Directory to logically represent a physical hardware device known as an IP gateway. The UM IP gateway can represent either an IP gateway or an IP PBX. The combination of the IP Gateway object and a UM Hunt Group object establishes a logical link between an IP gateway hardware device and a UM dial plan.
After the UM IP Gateway object is created, the IP gateway can be linked to or associated with a single or multiple UM hunt groups and UM dial plans. The UM hunt group provides a link between the UM IP gateway and a UM dial plan. By creating multiple UM hunt groups, you can associate a single UM IP gateway with multiple UM dial plans.
| Note: |
|---|
| Before an IP gateway can be used to process calls, a UM IP gateway must be associated with at least one UM dial plan. Also, at least one Unified Messaging server must be associated with at least one UM dial plan. |
Enabling and Disabling UM IP Gateways
By default, UM IP gateways are left in an enabled state after they are created. However, the UM IP gateway can be enabled or disabled. If you disable a UM IP gateway, it can be in one of two disabled modes. The first disabled mode forces all associated Unified Messaging servers to drop existing calls. The second disabled mode forces the Unified Messaging server that is associated with the UM IP gateway to stop handling any new calls presented by the IP gateway.
| Note: |
|---|
| If a UM IP Gateway object is deleted, the Unified Messaging servers associated with the UM IP gateway will no longer be able accept or process new call requests from the IP gateway. |
UM Hunt Groups
A hunt group describes a set of PBX or IP PBX resources or extension numbers that are shared by users. Hunt groups are used to efficiently distribute calls into or out of a given business unit. For example, a PBX or IP PBX might be configured to have 10 extension numbers for the sales department. The 10 sales extension numbers would be configured as one hunt group. In a PBX or IP PBX, hunt groups are used to efficiently locate an open line, extension, or channel when an incoming call is received.
In a telephony network, a hunt group is defined as a set of extension numbers that are grouped as a single logical unit. When an incoming call is received, the PBX or IP PBX uses the hunt group or the group of extensions that are defined to "hunt" for an available or open line, extension, or channel that can be used to receive the call.
There are multiple algorithms or methods that have been created to be used by a PBX or IP PBX to define how the open line, extension, or channel will be located. These include the following:
- Round robin
- Most idle
- Start with lowest number
Creating and defining a hunt group in a PBX or IP PBX reduces the possibility that a caller who places an incoming call will receive a busy signal when the call is received.
Understanding UM Hunt Groups
Unified Messaging hunt groups are very important to the operation of the Unified Messaging system. The UM hunt group is a logical representation of an existing PBX or IP PBX hunt group. UM hunt groups act as a connection or link between the UM IP gateway and the UM dial plan. A single UM hunt group is associated with at least one UM IP gateway and one UM dial plan.
Unified Messaging hunt groups are used to define the PBX or IP PBX hunt group from which incoming calls will be received. A pilot number that is defined for a hunt group in the PBX or IP PBX must also be defined within the UM hunt group. The pilot number is used to match the information presented for incoming calls through the Session Initiation Protocol (SIP) header on the message. The pilot number enables the Unified Messaging server to interpret the call together with the correct dial plan so that the voice or fax message can be routed correctly. It is very important to configure Unified Messaging hunt groups correctly, because incoming calls that do not correctly match the pilot number defined on the UM hunt group will not be answered and incoming call routing will fail.
When you create a Unified Messaging hunt group, you are enabling all Unified Messaging servers that are specified within the UM dial plan to communicate with an IP gateway. If you delete the Unified Messaging hunt group, the associated IP gateway will no longer service calls by using the specified pilot number. If the IP gateway is left without remaining UM hunt groups, the IP gateway will be unable to handle incoming calls.
UM Mailbox Policies
UM mailbox policies are required when you enable users for Exchange 2007 Unified Messaging. They are useful for applying and standardizing Unified Messaging configuration settings for UM-enabled users. You create UM mailbox policies to apply a common set of policies or security settings to a collection of UM-enabled mailboxes. You use Unified Messaging mailbox policies to set Unified Messaging settings for UM-enabled users, such as the following:
- PIN policies
- Dialing restrictions
- Other general UM mailbox policy properties
For example, you can create a UM mailbox policy to increase the level of PIN security by reducing the maximum number of logon failures before a user will be locked out for a specific group of UM-enabled users, such as executives.
Unified Messaging mailbox policies are created in the Configuration container in Active Directory by using the Exchange Management Shell or the Exchange Management Console. By default, a single UM mailbox policy is created every time that you create a UM dial plan. The new UM mailbox policy is associated with the UM dial plan and part of the dial plan name is included in the display name of the UM mailbox policy. However, you can create additional UM mailbox policies based on the needs of your organization. Although a single UM mailbox policy is required to enable users for Unified Messaging, you can create additional UM mailbox policies and apply a common set of mailbox policy settings for other groups of users.
The mailbox of each UM-enabled user must be linked to a single UM mailbox policy when the mailbox is enabled for Unified Messaging. After you create a UM mailbox policy, you link one or more UM-enabled mailboxes to the UM mailbox policy. This lets you control PIN security settings such as the minimum number of digits in a PIN or the maximum number of logon attempts for the UM-enabled users who are associated with the UM mailbox policy. If you prefer, you can also control message text settings or dialing restrictions for the same or a different group of UM-enabled mailboxes.
Multiple UM-enabled users can be linked to a single UM mailbox policy. However, a single user can be associated with only one UM mailbox policy. After you create a new UM mailbox policy and link it to a UM dial plan, the UM mailbox policy settings that are defined are applied to the UM-enabled users. The settings that are defined on a UM dial plan and a UM mailbox policy will be applied to all users who are associated with the UM mailbox policy.
The following figure illustrates how Unified Messaging mailbox policies can be created to control dialing restrictions and PIN security settings for three different groups.
UM Auto Attendants
Exchange 2007 Unified Messaging enables you to create one or more UM auto attendants, depending on the needs of your organization. UM auto attendants can be used to create a voice menu system for an organization that lets external and internal callers move through the UM auto attendant menu system to locate and place or transfer calls to company users or departments in an organization.
When anonymous or unauthenticated users call an external business telephone number, or when internal callers call a defined extension number, they are presented with a series of voice prompts that help them place a call to a user or locate a user in the organization and then place a call to that user. The UM auto attendant is a series of voice prompts or .wav files that callers hear instead of a human operator when they call an organization that has Exchange 2007 Unified Messaging. The UM auto attendant lets callers move through the menu system, place calls, or locate users by using DTMF or voice inputs. However, for automatic speech recognition (ASR) or voice inputs to be used, you must enable ASR on the UM auto attendant.
| Important: |
|---|
| In some companies (especially in East Asia), office telephones may not have letters on the keys of the telephone. This makes the spell-the-name feature that uses the DTMF interface almost impossible without a working knowledge of the key mappings. By default, Exchange 2007 Unified Messaging uses the E.161 key mapping. For example, 2=ABC, 3=DEF, 4=GHI, 5=JKL, 6=MNO, 7=PQRS, 8=TUV, and 9=WXYZ. When a combination of letters and numbers is inputted, for example "Mike1092", the numeric digits are mapped to themselves. For an e-mail alias of "Mike1092" to be entered correctly, the user must press the numbers 64531092. Also, there will not be a telephone key equivalent for characters other than A-Z and 0-9. Therefore, these characters should not be entered. For example, the e-mail alias "mike.wilson" would be entered as 6453945766. Even though there are 11 characters to be input, only 10 digits are entered by the user because the period (.) does not have a digit equivalent. |
A UM auto attendant has the following features:
- It provides corporate or informational greetings.
- It provides custom corporate menus. You can link a menu option
to another auto attendant to have more than one level.
- It provides a directory search function that enables a caller
to search an organization's directory for a name.
- It enables a caller to connect to the telephone of, or leave a
message for, members of the organization.
In Active Directory, each UM auto attendant that is created is represented as an object. There is no limit to the number of UM auto attendants that you can create in Active Directory. Each auto attendant can support an unlimited number of extensions. A UM auto attendant is associated with one, and only one, UM dial plan. However, UM auto attendants can reference or link to other UM auto attendants.
An incoming call that is received from an external telephone number or an internal telephone extension is processed by a Unified Messaging server and then sent to a UM auto attendant that has been created. The UM auto attendant is configured by the system administrator to use prerecorded voice (.wav) files that are then played over the telephone to the caller and that enable the caller to move through the Unified Messaging menu system. When you configure a UM auto attendant, you can customize all the .wav files that are used to meet the needs of your organization. For more information about custom prompts in Unified Messaging, see Understanding Unified Messaging Audio Prompts.
For more information about message flow with UM auto attendants, see Unified Messaging Auto Attendant Call Processing.
Auto Attendant with Multiple Languages
There are situations in which you may have to provide callers with auto attendants that have different languages. The language setting that is available on a UM auto attendant enables you to configure the default prompt language on the auto attendant. When you are using the default system prompts for the auto attendant, the default prompt language is the language that the caller will hear when the auto attendant answers the incoming call. This language setting will affect only the default system prompts that are provided when the Unified Messaging server role is installed. This setting will not affect custom prompts that have been configured on an auto attendant. The language that is selected as the default for the auto attendant is based on the version of Exchange 2007 that is installed.
When you install the U.S.-English version of Exchange 2007, there will be only one language available to configure on UM auto attendants: U.S. English. However, if you install a localized version of Exchange 2007, for example, Japanese, you will be able to configure the auto attendant that you create to use Japanese or U.S. English as the default language. Additional UM language packs can be installed on a Unified Messaging server to enable you to use other default language options on an auto attendant.
 Caution: |
|---|
| You cannot install UM language packs by using the .msi file for the language. |
For example, if you have a business that is based in the United States but requires a menu system that gives callers the options of moving through the system in U.S. English, Spanish, and French, you must first install the UM language packs that you need. In this case, if you have installed the U.S.-English version of Exchange 2007, you would install the UM language packs for Spanish and French. However, because a Unified Messaging auto attendant can have only one language configured at a time, you would create four auto attendants: a main auto attendant that is configured to use U.S. English and then one auto attendant for each language: U.S. English, Spanish, and French. You would then configure the main auto attendant to have the appropriate key mappings to access the other auto attendants that you have created for each language. In this example, the main auto attendant would answer the incoming call and the caller would hear, "Welcome to Contoso, Ltd. For English, press or say 1. For Spanish, press or say 2. For French, press or say 3."
Auto Attendant Examples
The following examples demonstrate how you can use UM auto attendants together with Exchange 2007 Unified Messaging:
- Example 1 At a company called Contoso,
Ltd., external customers can use three external telephone numbers:
425-555-1111 (Corporate Offices), 425-555-2222 (Product Support),
and 425-555-3333 (Sales). The Human Resources, Administration, and
Accounting departments have internal telephone extensions and must
be accessed from the Corporate Offices UM auto attendant.
To create a UM auto attendant structure that supports this scenario, create and configure three UM auto attendants that have the appropriate external telephone numbers. Create three other UM auto attendants for each department in the Corporate Offices. Then you configure each UM auto attendant based on your requirements, such as the greeting type or other navigational information.
The following figure is a graphical representation of how Unified Messaging auto attendants can be used in Example 1.
How to configure multiple UM auto attendants with multiple outside business telephone lines - Example 2 At a company called Contoso,
Ltd., external customers call one main number for the business,
425-555-1000. When an external caller calls the main number, the UM
auto attendant answers and prompts the caller by saying, "Welcome
to Contoso, Ltd. Please press or say 'One' to be transferred to
corporate administration. Please press or say 'Two' to be
transferred to product support. Please press or say 'Three' to be
transferred to corporate information. Please press or say 'Zero' to
be transferred to the operator." To create a UM auto attendant
structure that supports this scenario, you create a UM auto
attendant that has customized extensions that route the call to the
appropriate extension number.
The following figure is a graphical representation of how Unified Messaging auto attendants can be used in Example 2.
How to configure multiple UM auto attendants with a single outside business telephone line
Unified Messaging Call
Answering
This section describes how Exchange 2007 Unified Messaging handles message flow in different incoming call scenarios.
Exchange 2007 Unified Messaging handles the following types of incoming calls:
- Voice
- Fax
- Outlook Voice Access
- Play on Phone
- Auto attendant
Note:Call handling is a term that describes how incoming calls are answered and handled by a Unified Messaging server.
Exchange 2007 Unified Messaging depends on Active Directory to route incoming calls. For call answering to function correctly, each UM-enabled recipient must have a telephone extension number listed in Active Directory. The extension number for the recipient is listed in Active Directory and is mapped to the extension number that is configured on the user's UM-enabled Exchange mailbox. When a Unified Messaging server answers a call, an Active Directory lookup is performed to locate the appropriate UM-enabled recipient, and then the message is routed to the recipient's mailbox.
Message Flow
Message flow in Unified Messaging is the process by which a message that is received by a Unified Messaging server is routed in an Exchange 2007 organization.
| Note: |
|---|
| In earlier versions of Microsoft Exchange, routing groups were used to route messages between bridgehead servers. In Exchange 2007, bridgehead servers are known as Hub Transport servers. There are no routing groups in Exchange 2007. |
For example, in an incoming call scenario that includes incoming voice and fax messages, a Unified Messaging server uses the SMTP transport to submit the voice or fax message to the Exchange 2007 server that has the Hub Transport server role installed. In a routing scenario that includes multiple Hub Transport servers, the incoming voice or fax mail message is first submitted to the closest Hub Transport server and is then routed to the appropriate Mailbox server that contains the UM-enabled mailbox.
| Note: |
|---|
| To make sure that all incoming messages are transmitted and delivered to UM-enabled recipients, the Unified Messaging servers use a spooling or retry algorithm. The Unified Messaging servers try to connect to a Hub Transport server every 30 seconds to submit all messages that are stored on the Unified Messaging server. |
Voice and Fax Call Processing
Voice and fax calls that come in to an Exchange 2007 organization can be placed by callers who are inside or outside the organization. When a caller places a call to a UM-enabled user's telephone extension and the user is unavailable to answer the call, the PBX forwards or routes the incoming call to an IP gateway and then to the Unified Messaging server. In a Unified Messaging system that uses an IP PBX, the IP PBX forwards the incoming message to the Unified Messaging server. The IP gateway or the IP PBX translates or converts the incoming audio stream into RTP for incoming voice messages or the T.38 protocol for incoming fax messages. The stream of IP data is then passed to the Unified Messaging server. After the Unified Messaging server receives the call, the Unified Messaging server processes the message and determines how to route the message.
The following figure illustrates how incoming voice and fax messages flow in an Exchange 2007 organization.
In the previous example, a call arrives at the PBX and is then forwarded to the Unified Messaging pilot number. The pilot number is mapped to the hunt group that goes to the IP gateway. The IP gateway presents the call to the Unified Messaging server. The Unified Messaging server then receives SIP invitation header information such as who the call is for, who the caller is, and why the call was redirected. The Unified Messaging server then looks up the number that was called and identifies which user was called and which dial plan the user belongs to. From there, the Unified Messaging server looks in Active Directory to see whether the user is enabled for UM. If the user is enabled for UM, the caller receives a greeting for the user. Users' greetings are securely stored in their mailbox. This lets them manage their greetings. After the caller reaches the user's voice mail greeting, they can leave a voice message that will be sent to the user's mailbox. The Unified Messaging server then records the message, locates a Hub Transport server, and submits the voice message to the Mailbox server that contains the UM-enabled user's mailbox.
UM Auto Attendant Message Processing
When external or anonymous callers place a call by using an external business telephone number, or an internal anonymous caller places a call to an internal extension number, they are presented with voice prompts to help them move through the Unified Messaging menu system. The UM auto attendant is a set of voice prompts or .wav files that are played to callers instead of a human operator or receptionist when they call in to an organization that has Exchange 2007 Unified Messaging. Exchange 2007 Unified Messaging enables you to create one or more auto attendants, depending on the needs of your organization.
Auto Attendant Message Flow
When a call is received by a Unified Messaging server, the Unified Messaging server performs a Lightweight Directory Access Protocol (LDAP) query to an Active Directory domain controller to determine how to handle the incoming call.
The following figure illustrates the message flow when Unified Messaging auto attendants are used in an Exchange 2007 organization.
After you have created and enabled the auto attendant for your organization and associated it with a UM dial plan, the auto attendant can start to process calls. When a call for an auto attendant is received, it is first processed by the IP gateway and mapped to the appropriate hunt group. The call is then sent to the Unified Messaging server. The Unified Messaging server looks up the auto attendant object in Active Directory and then provides the correct auto attendant to handle the call.
UM Outlook Voice Access Call Processing
With Exchange 2007 Unified Messaging, UM-enabled users or subscribers can access their e-mail, contacts, and calendaring information by using a standard analog, digital, or cellular telephone. When a UM-enabled user uses Outlook Voice Access, they can perform the following tasks:
- Listen to new and saved e-mail and voice mail messages.
- Forward, reply to, save, and delete e-mail and voice mail
messages.
- Interact with their calendar.
- Locate a person in the global address list (GAL) or their
personal contacts.
- Send a voice message to a person.
- Change their PIN, spoken name, or greetings.
Outlook Voice Access Message Flow
Incoming calls that are received by a Unified Messaging server from an Outlook Voice Access user are only routed to a Mailbox server to enable the user to access their mailbox. However, if a message is submitted by using Outlook Voice Access, for example, a change in the schedule of a meeting, the message is first submitted to a Hub Transport server within the same Active Directory site as the Unified Messaging server before it is routed to the recipient's mailbox.
The following figure illustrates how incoming calls and messages placed by subscribers or UM-enabled users flow in an Exchange 2007 organization.
When a user calls in to Outlook Voice Access, the PBX receives the call for the Unified Messaging number and the IP gateway presents the call to the Unified Messaging server. The Unified Messaging server then looks up the user account information in Active Directory and gathers information about the user. This information includes dial plan information. If the user places the call from their own extension, they are prompted for their PIN. The Unified Messaging server contacts the Mailbox server and authenticates the user. From there, the Unified Messaging server and the Mailbox server communicate information back to the user, as requested.
Unified Messaging Server
Scalability
Scalability is defined as the capability to increase resources to increase the capacity of a given service. There are two types of scalability that can be used to increase the capacity of Unified Messaging servers in your organization: horizontal and vertical. In Exchange 2007 Unified Messaging, when you scale vertically, you add hardware resources to a single Unified Messaging server or multiple Unified Messaging servers, such as the following:
- Adding more hard disk space for message storage
- Increasing the speed or number of processors
- Increasing the amount or speed of RAM
- Increasing the number of network adapters or increasing the
number of local area network (LAN) ports in a single network
adapter
In Unified Messaging, when you scale horizontally, you install the Unified Messaging server role on new Unified Messaging servers and add more Unified Messaging servers to a dial plan to increase the number of incoming concurrent calls that the system can accept. To scale your Unified Messaging environment horizontally, you can also increase the number of IP gateways. This increases the number of ports that are available to be used for incoming calls.
Exchange 2007 Unified Messaging provides an efficient and simple deployment model that is highly scalable without increasing the complexity of the deployment. There are many deployment models for Unified Messaging in your organization. But the recommended deployment model for Unified Messaging is to centralize your Unified Messaging servers. All the available deployment options for Unified Messaging have several steps in common that are required to create a scalable system to support large numbers of Unified Messaging users. These steps are the following:
- Provision PBX lines The first step in
building a highly scalable UM solution is to provision PBX
lines.
- Organize channels After you have
provisioned PBX-based voice channels, you can organize the channels
as hunt groups.
- Deploy IP gateways After you have
organized your voice channels as hunt groups, you end these
channels at IP gateways. IP gateways are used with a legacy PBX to
convert the circuit-switched protocols found on a telephony network
to IP-based packet-switched protocols.
- Add more Unified Messaging servers to a dial
plan If you have to increase the number of
calls that can be handled by Unified Messaging, you can install and
configure additional Unified Messaging servers and add them to a
dial plan. In most cases, IP gateways will use DNS to load balance
between the existing Unified Messaging servers and the additional
Unified Messaging servers that have been installed.
Network Traffic
Every incoming call that is received from an IP gateway will generate IP-based network traffic and will consume some amount of your available network bandwidth. Prior to deploying Unified Messaging, you should perform an analysis of the network traffic to determine current usage patterns and identify any potential issues. On most networks, bandwidth demand is not evenly distributed throughout business hours. Because all the IP-based calls are routed directly to your Unified Messaging servers from the IP gateways on your network and this IP-based network traffic consumes some available bandwidth, you should follow these recommendations and guidelines:
- Place your PBXs physically close to your IP gateways.
- Place your IP gateways and your Unified Messaging servers on
the same well connected network or within the same physical
site.
- Place your Unified Messaging servers on the same well connected
network or within the same physical site as other computers that
have Exchange 2007 server roles installed, including Mailbox,
Hub Transport, and Client Access servers.
- Terminate your Wide Area Network (WAN) connections close to
where your telephony equipment is located.
- In branch office scenarios or over WAN connections, use the
G.723.1 codec instead of the G.711u or G.711A codec to minimize the
network traffic that is passed between your IP gateways and your
Unified Messaging servers.
Unified Messaging Servers
Generally, Unified Messaging scalability is determined by the number of concurrent calls. By default, a single Unified Messaging server can accept a maximum of 100 concurrent voice calls and 100 concurrent incoming fax calls. These calls can be either incoming or outgoing and can be generated when a user leaves a voice mail message, when an Outlook Voice Access user accesses their Exchange 2007 mailbox, or by a user who uses the Play on Phone feature to listen to their voice messages. Although the number of concurrent calls is an important factor to consider when you build a scalable Unified Messaging infrastructure, you must also determine the best codec to use to encode the voice messages and the types and number of users who you have to support.
- Number of concurrent calls Although, by
default, a Unified Messaging server can accept 100 concurrent voice
messages and 100 concurrent fax messages, a single Unified
Messaging server can be configured to accept a maximum of 200
concurrent voice messages and 200 concurrent incoming fax messages.
When you increase the number of concurrent connections on a single
Unified Messaging server, more system resources are required than
if you decrease the number of concurrent connections. It is
especially important to decrease this setting on low-end, slower
computers on which the Unified Messaging server is installed.
Performance counters are available and the Get-UMActiveCalls
cmdlet can also be used to monitor the current number of concurrent
calls that are connected to a Unified Messaging server.
If the number of concurrent calls required by your organization is larger than the number that is supported by a single Unified Messaging server, you can scale horizontally and increase the capacity of concurrent calls by installing the Unified Messaging server role and then adding the Unified Messaging server to a dial plan.
- Voice mail storage codec The term
"codec" is a combination of the words "coding" and "decoding" and
relates to digital data. A codec is a computer program or software
that transforms digital data into an audio file format or streaming
audio format.
In Exchange Unified Messaging, there are two general types of codecs: the codec that is used between IP gateways and the codec that is used to encode voice messages. The Windows Media Audio (WMA), Group System Mobile (GSM) 06.10, and G.711 Pulse Code Modulation (PCM) Linear audio codecs are used to create .wma and .wav audio files for voice messages. However, the file type that is used depends on the audio codec that is used to create the voice message audio file. In Exchange Unified Messaging, the .wma audio codec creates .wma audio files and the GSM 06.10 and G.711 PCM Linear audio codecs produce .wav audio files. However, depending on the codec that is used, an audio file in .wma or .wav format is sent together with the e-mail message to the intended voice mail recipient. The size of Unified Messaging voice messages depends on the size of the attachment that holds the voice data. Additionally, the size of the attachment depends on the following factors:
- The duration of the voice mail recording
- The audio codec that is used
- The audio file storage format
Note:In the following figure, the average length of a call-answered voice message is approximately 30 seconds.
The sample bit rate (bit/sec) and compression
properties for each audio codec that is used in Unified Messaging
are as follows:
- WMA – 16-bit – compressed file
- G.711 – 16-bit – uncompressed file
- GSM 06.10 – 8-bit – compressed file
Generally, you should allow for the following number of concurrent calls per Unified Messaging server:
- 60 if the default dial plan codec is WMA
- 75 if the default dial plan codec is GSM
Audio file size - The duration of the voice mail recording
- Types of users There are two types of
users who access the Unified Messaging system: authenticated users
and unauthenticated users. These two types of users consume Unified
Messaging resources. When you build a scalable Unified Messaging
environment, you must consider the effect that these users will
have and the resources that each of these users will consume.
- Authenticated Authenticated users are
UM-enabled and can access their mailbox by using Outlook Voice
Access. Authenticated users consume Unified Messaging server
resources in several ways, including by directly calling in to a
subscriber access number, logging on to their mailboxes, accessing
their messages, calendar, contacts or the directory, and by using a
Unified Messaging server to play voice messages over a telephone
using the Play on Phone feature. They can also indirectly consume
resources by transferring a call, sending a voice message, or
calling a user's extension number and leaving a voice message or
sending a fax.
- Unauthenticated Users who call in to a
Unified Messaging auto attendant or call in to a subscriber access
number but do not log on to their mailbox are unauthenticated
callers. However, Unified Messaging resources are used to service
their requests. Every time that a caller calls in to a UM auto
attendant or uses a subscriber access number but does not log on to
their mailbox, they still consume resources by transferring a call,
sending a voice message, transferring to another auto attendant,
transferring to another telephone number, or listening to recorded
audio prompts.
- Authenticated Authenticated users are
UM-enabled and can access their mailbox by using Outlook Voice
Access. Authenticated users consume Unified Messaging server
resources in several ways, including by directly calling in to a
subscriber access number, logging on to their mailboxes, accessing
their messages, calendar, contacts or the directory, and by using a
Unified Messaging server to play voice messages over a telephone
using the Play on Phone feature. They can also indirectly consume
resources by transferring a call, sending a voice message, or
calling a user's extension number and leaving a voice message or
sending a fax.
IP Gateways
Unified Messaging relies on the ability of the IP gateway to translate TDM or telephony circuit-switched based protocols, such as Integrated Services Digital Network (ISDN) or QSIG, from a PBX to protocols based on VoIP or IP, such as SIP, RTP, or T.38 for real-time facsimile transport.
IP gateways are available from multiple manufacturers in sizes and models that range from 4 ports to 32 ports. You can deploy as many IP gateways as necessary to provide for capacity and fault tolerance. If the number of calls or ports that are required is larger than the number of calls or ports that are supported by a single IP gateway, you can scale horizontally and increase the number of calls that can be accepted or the number of ports by installing and configuring additional IP gateways, creating the UM IP gateway object, and configuring the appropriate hunt groups to support your environment.
It is equally important to match the number of IP gateways that you have in your environment to the number of Unified Messaging servers that are available. For example, you should not configure 10 IP gateways that are each connected with a T-1 line to a single Unified Messaging server. This would mean that the Unified Messaging server would have to support 240 concurrent incoming calls. You should consider this and scale your IP gateways to Unified Messaging servers appropriately.
Unified Messaging
Availability
Minimum uptime requirements may vary among organizations. However, every organization wants to achieve a high level of uptime, especially for their telephony system. An organization's telephony system is frequently business-critical and must be highly available to users. One of the factors that you must consider when you deploy a Unified Messaging system is the ability for the system to provide services for users when a key component such as a Unified Messaging server or an IP gateway becomes unavailable.
- Unified Messaging servers In
Exchange 2007, Unified Messaging is designed to run as a
service and a worker process. This means that, if the service is
using lots of system resources or has become unresponsive, the
worker process can be recycled. The UM worker process is
responsible for dealing with outages in Mailbox servers, Hub
Transport servers, and domain controllers. If, for example, a
Mailbox server for a user is unavailable, the Unified Messaging
server will continue to accept calls on behalf of the
user. However, the user's custom greeting will not be played.
Instead, a standard greeting will be used for calls to that user.
Additionally, if the Hub Transport server is unavailable, the
Unified Messaging server will continue to accept calls and queue
the calls, depending on how you have configured the queuing limit,
until the Hub Transport server is available. However, in a
situation where all domain controllers are unavailable, the Unified
Messaging server will be unable to accept calls.
Unified Messaging deployments can be made more resilient and more available by adding Unified Messaging servers to a single dial plan in an N+1 configuration. This means that, if you need two Unified Messaging servers, you would install and configure an additional Unified Messaging server so that you would have a Unified Messaging server to take the place of a Unified Messaging server that is failing or must be taken offline.
When you add multiple servers to a single dial plan, the IP gateway will try to connect to a Unified Messaging server that is listed in the configuration on the IP gateway by IP address, or by fully qualified domain name (FQDN) for TLS deployments. If the Unified Messaging server is unavailable, the IP gateway will try to connect to the Unified Messaging server again after 5 seconds. If there is no response from the Unified Messaging server, the IP gateway will try to connect to the next Unified Messaging server in the list that is configured on the IP gateway.
- IP gateways If you want to create
redundancy to provide for IP gateway availability, you should add
multiple IP gateways in an N+1 configuration. This means that, if
you need two IP gateways, you would install and configure an
additional IP gateway so that you would have an IP gateway to take
the place of an IP gateway that is failing or must be taken
offline.
You must configure your PBX so that it will send the incoming calls that it has received to different IP gateways. After you configure the PBX, the PBX will detect a failure or no signal or that a port is not answering call. This will enable the PBX to redirect calls to an IP gateway that can answer incoming calls.
The IP gateways that are supported by Unified Messaging can be configured to route calls to Unified Messaging servers in a round-robin manner. To enable an IP gateway, you must configure each IP gateway with the IP addresses for your Unified Messaging servers that will be answering calls from the IP gateway. These are the Unified Messaging servers that are associated with the same dial plan as the UM IP gateway object that logically represents the IP gateway. This will enable all the UM IP gateways to forward incoming calls to the Unified Messaging servers that are associated with the same dial plan. Then, if an IP gateway fails, the PBX will send the call to an IP gateway that can answer the call. The IP gateway, in turn, will forward the call to a Unified Messaging server within the same dial plan. If the call is sent to a Unified Messaging server that is not available, the IP gateway will try to contact the Unified Messaging server again. If it is unsuccessful in contacting the Unified Messaging server, it will then use the next Unified Messaging server in the list that is configured on the IP gateway to answer the call. However, not all supported IP gateways can be configured to support both load balancing and to be able to detect if a server has been taken offline or is failing.
Load Balancing in Unified
Messaging
Exchange 2007 Unified Messaging deployments can be made more resilient by deploying multiple Unified Messaging servers to a single dial plan to balance the load of incoming calls. The IP gateways that are supported by Unified Messaging can be configured to route calls in a round-robin manner to balance the load between multiple Unified Messaging servers that are within a dial plan.
Round robin is a method for distributing the workload among multiple servers. However, round robin does not by itself enable an IP gateway to detect a server failure. If one of the Unified Messaging servers fails and if the IP gateway cannot detect that a Unified Messaging server is unavailable, the IP gateway will continue to send incoming calls to the Unified Messaging server until you detect the failure and remove the server from the dial plan. After you remove the Unified Messaging server from the dial plan, you should also remove the IP address or FQDN for the Unified Messaging server from the configuration on the IP gateway.
Unified Messaging does not use round-robin DNS or Network Load Balancing (NLB) to distribute incoming calls. Round-robin DNS can be used on multi-homed computers and can be used to distribute the load for other services, but not for Unified Messaging. NLB is used with other services to distribute client requests and to automatically detect whether a server is unavailable, and then to redistribute other client requests to the remaining server. However, it also cannot be used with Unified Messaging. The only way to distribute or balance the load between Unified Messaging servers in a dial plan is for the IP gateway to be configured with the IP addresses or FQDNs of the Unified Messaging servers in the dial plan. The IP gateway will use the list to distribute the load across all the Unified Messaging servers in the dial plan and can also detect a server failure if the IP supports this functionality.
Another way to load balance your Unified Messaging deployment is to configure PBX hunt groups to connect to multiple IP gateways and then configure the hunt groups to load balance across the IP gateways.
For More Information
For more information about Exchange 2007 Unified Messaging, see the following resources: