Two new IP-PBXs (Cisco CUCM 4.2, 5.1 and 6.1 and Seltatel) have qualified in the Open Interoperability program here. The new support adds support only for DirectSIP but not Dual Forking. Because of the enabled functionality for DirectSIP, you no longer need a supported Voice Gateway between an OCS Mediation Server and a Qualified IP-PBX that supports DirectSIP.
What does this mean to you? You can connect the IP-PBX to the OCS Mediation Server via Direct SIP and have your supported IP-PBX trunk the DID numbers you will be using for Enterprise Voice directly to OCS. This is Standalone Mode.
As time goes on, vendors will add Dual Forking support to their IP-PBXs and become qualified in the Open Interoperability Program. An example is Cisco CUCM 7 which adds support for Dual Forking (Coexistence Mode). Dual Forking allows an incoming call to be forked to OCS. This means that you can share an extension between your hard phone and OCS. When an incoming call comes in, you can have CUCM 7 ring your hard phone as well as trunk the call to OCS. OCS will then ring all SIP enabled endpoints. When one end point picks up the phone, the IP-PBX and OCS know how to talk to each other to let them know that an endpoint has picked up and all other endpoints stop ringing.
For outbound, it’s similar in the fashion that OCS and CUCM (and other IP-PBXs that support Dual Forking) will know how to talk to each other so that when OCS places an outbound call, it will ring the other user’s SIP enabled endpoints as well as tell CUCM to ring the user’s hard phone. Just as with incoming calls, when a user picks up an endpoint, all other endpoints stop ringing.