Consider the following scenario:
A user makes a Public Switched Telephone Network (PSTN) call In a Microsoft Lync Server 2010 environment..
The PSTN call is routed from a Mediation server to a gateway that is disconnected from the Mediation server.
In this scenario, the Mediation server rejects the call. Additionally, the Mediation server returns the following error message:
SIP/2.0 503 Service Unavailable
Note This error message only contains an "ms-trunking-peer-state" header. However, it should also contain an "ms-trunking-peer" header.
To resolve this issue, install the following cumulative update:
2502810 Description of the cumulative update for Lync Server 2010, Mediation server: April 2011
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.