You have a Microsoft Lync Server 2010 environment that contains a Public Switched Telephone Network (PSTN) gateway, and a virtual PSTN gateway that has the same IP address as the PSTN Gateway.
The virtual PSTN gateway is associated to a Media Termination Point (MTP) resource that has a different IP address from the PSTN gateway signaling IP address. Additionally, the MTP resource is located in a different site.
You enable media bypass on the trunk connection for all the sites in the Lync Server 2010 environment.
A PSTN user makes a call that uses MTP in a remote site to a Lync 2010 user.
Note The call is routed to the Lync 2010 user through the virtual PSTN gateway.
In this scenario, Mediation Server may sends the bypass ID associated to the PSTN gateway instead of the virtual PSTN gateway.
This issue occurs because the INVITE request arrives from the same signaling IP address. Therefore Mediation Server cannot determine reliably whether the call comes from the PSTN gateway, or the virtual PSTN gateway.
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.
For more information about how Media Bypase works, visit the following Microsoft Technet website: