The OCS team has now posted more information on Direct SIP integration with third-party PBX’s and the RFC 3966 “changes” made to accomplish this that I started to discuss in my post Cisco is now a qualified IP PBX for Direct SIP with OCS 2007. Here’s a snippet -
With the hotfixes released in August 2008, Microsoft is significantly improving the capability for Office Communications Server 2007 to exchange calls with SIP-based IP-PBX, in particular from Cisco. As a consequence, Microsoft now supports OCS deployments in Direct SIP with IP-PBX between Office Communications Server and specific versions of Cisco Call Manager.
This change gives administrators the possibility to set up Office Communications Server so that it can directly interoperate with IP-PBX using E.164 globally routable telephone numbers without RFC3966’s mandated “+” prefix. Additionally, Office Communications Server will now be able to interoperate with IP-PBX within a private dialing plan, exchanging locally routable private numbers.
The post continues to discuss these changes in details and is certainly a recommended read. Find it here.