Most IP-PBXs support dual forking, which enables sharing of a common number with Office Communications Server 2007 R2. Additionally, the PBX could also support Direct SIP for certain numbers (managed by Office Communications Server), or manage the numbers themselves (for users not enabled for voice by using Office Communications Server 2007 R2). An extension of this concept is dual forking with RCC. RCC provides information to Office Communications Server 2007 R2 about the state of PBX-connected lines. This allows presence updates when the user is on the PBX-connected station set.

The following figure demonstrates dual forking (TelephonyMode=3).

As shown in the figure, the following activities occur:

Step 1:User B places a PBX phone direct call to PBX Phone User A. The PBX phone for User A rings. The PBX phone can be answered. Additionally, the call is forked to a Mediation Server and the call simultaneously displays a notification on the Communicator client for User A. The call can be answered by using Communicator.

Step 2:A Communicator call is placed from User A to User B. The Communicator client for User B receives a notification to receive the call from User A. Simultaneously, the PBX phone for User B rings.