A Microsoft Office Communications Server 2007 R2 Mediation server connects to the Public Switched Telephone Network (PSTN) through a Session Initiation Protocol (SIP) trunk that is supported by a Session Border Controller. In this situation, Office Communicator 2007 R2 outgoing calls fail.
This issue occurs because the UPDATE request that is sent by Office Communications Server 2007 R2 does not contain a contact header. The UPDATE request is to keep the call alive. However, in Request for Comments (RFC) 3311, a contact header must be stamped for a UPDATE request. Therefore, the Session Border Controller does not pass the UPDATE request.
To resolve this problem, install the following cumulative update:
2291724 Description of the cumulative update for Office Communications Server 2007 R2, Unified Communications Managed API 2.0 Core Redist 64-bit: November 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.