In the standalone scenario, users in an organization replace their legacy PBX phones with Office Communicator 2007 R2 or an Office Communications Server 2007 R2 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.
A peer-to-peer call using Communicator 2007 R2 is when one Communicator 2007 R2 client places an Office Communications Server 2007 R2 call to another Communicator 2007 R2 client. In this case, the call is a direct VoIP, it is handled exclusively by the server, and the presence status (in a call) is broadcast to other Communicator clients. The call is a SIP INVITE from one client to another and audio is established.
Be aware that when a PSTN gateway and Mediation Server are present, PSTN calls can be made to Communicator clients and Communicator clients can place calls to the outside PSTN network.
The following figure shows TelephonyMode=1 when a Communicator 2007 R2 client (Client A) places a peer-to-peer call with another Communicator 2007 R2 client (Client B).