Microsoft Office Communicator 2007 R2 calls may fail when the calls are routed to the Public Switched Telephone Network (PSTN).
This issue occurs because Outbound Routing processes Windows Management Instrumentation (WMI) events on an incoming request thread. The call is then held while the WMI events are processed. This behavior may cause a time out. Therefore, the Office Communicator 2007 R2, Mediation server does not receive the information from Outbound Routing, and then the call fails.
To resolve this problem, install the following cumulative update:
2404547 Description of cumulative update for Office Communications Server 2007 R2: November, 2010
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.