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Microsoft Office Communications Server 2007 Mediation Server sends the following generic SIP response code if the gateway to which Mediation Server has routed the outgoing call is unreachable because of connectivity issues:
500: Server Internal Error
This generic response code does not trigger the retry logic in Communications Server 2007.
To resolve this problem, download and install the Communications Server 2007 Mediation Server update package that was released in October 2008.
For more information about the update package, click the following article number to view the article in the Microsoft Knowledge Base:
956829 Description of the update package for Communications Server 2007 Mediation Server: October 2008
After you install this update package, the Mediation Server sends an SIP response code "504: Server Time-out" to Office Communications Server 2007 if the gateway is unreachable because of connectivity issues. A “504” SIP response code forces Office Communications Server 2007 to find an alternative route for the outgoing call, if it is possible.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.