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.
Read more at source: http://communicationsserverteam.com/archive/2008/09/26/270.aspx