"SIP/2.0 503 Service Unavailable" error message when you make a PSTN call in Lync Server 2010

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Lync Server 2010 Enterprise Edition Lync Server 2010 Standard Edition More...Less

Symptoms

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.

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