Office Communications Server 2007 R2 can work with existing telephony infrastructure. With Office Communications Server 2007 R2, users can not only send and receive calls with the Public Switched Telephone Network (PSTN), but can integrate with a company's Private Branch Exchange (PBX) system.

Office Communications Server 2007 R2 also combines VoIP with open standards to make the telephone menu-based part of the unified communications strategy. This extends the integration features that bridge the VoIP standards supported by Office Communications Server 2007 R2 with implementations that use older standards.

At the center of this integration is the Mediation Server role of Office Communications Server 2007 R2. It provides a single interface and uses open-standard SIP for signaling interoperability. Mediation Server takes calls from third-party IP-PBX systems or SIP/PSTN Gateways and moves them onto the network using the adaptive codec, remote user, and security models that are the basis for call setup and media with Office Communications Server 2007 R2.

The following two configurations are common Office Communications Server 2007 R2 call scenarios.

• Standalone   Users in an organization replace their legacy phones with Office Communicator 2007 R2 or an Office Communications Server 2007 IP phone. This way, users who are configured to use Office Communications Server 2007 R2 will make and receive calls with Office Communicator 2007 R2, and users configured to use the PBX will make and receive calls using the PBX. Each group continues to have a smooth calling experience, including extension-based dialing capability. This scenario works especially well for mobile employees. The standalone scenario can be implemented by using either a SIP/PSTN gateway, or a direct SIP connection to the PBX.
  
  
• Co-existence   An incoming call is forked between the PBX phone and the user's logged-in Office Communicator 2007 R2 endpoints. Both the PBX phone and Office Communicator 2007 R2 co-exist on the user's desktop. The co-existence scenario is implemented by using a specification called Dual Forking, and can be deployed with or without remote call control (RCC).
  
  

This section covers three scenarios for calling as follows:

• Standalone Communicator (TelephonyMode=1)
  
  
• Co-existence of Office Communicator and PBX (TelephonyMode=2)
  
  
• Dual forking (TelephonyMode=3)