You have a Microsoft Lync Server 2010 environment that has more than one Mediation server and a Public Switched Telephone Network (PSTN) gateway deployed. The Mediation servers are configured with dual Network Interface Cards (NICs).
A Mediation server (Mediation server A) cannot connect to the PSTN gateway because of a network issue that is specific to Mediation server A.
You call a PSTN number by using Lync 2010.
The call is routed to Mediation server A.
In this scenario, Mediation server A returns a SIP/2.0 504 Service Unavailable error message that contains the following header:
Additionally, any PSTN calls that you make afterward are rejected.
This issue occurs because an ms-trunking-peer-state: down header is added to all calls after the Mediation server detects lost connection to the PSTN gateway. When the Outbound Route application on Lync Server 2010 receives this header, it assumes that the gateway is offline, and then rejects any calls afterward. Additionally, it does not try to route calls to a different Mediation server.
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.