On a computer that is running Microsoft Office Communications Server 2007 R2 and that has the Mediation Server server role installed, a Mediation Server does not send a response to the Public Switched Telephone Network (PSTN) gateway when the Mediation Server cannot contact the front-end server.
To fix this problem, install the following update:
977937 Description of the cumulative update for Office Communications Server 2007 R2, Mediation Server: April 2010
Note After you apply this update, the Mediation Server will send a 503 final response to the PSTN gateway when it cannot contact the front-end server.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.