Consider the following scenario:
You enable the Collocated Mediation Server option in a Microsoft Lync Server 2010 Front End Server pool in a Lync Server 2010 environment.
You deploy a Front End Server in the Lync Server 2010 Front End Server pool.
You enable the Limit service usage to selected IP address option, and then you configure the Primary IP address and the PSTN IP address of the Front End Server.
You register the Primary IP address on the DNS server. However, you do not register the PSTN IP address on the DNS server.
You deploy the Mobility service in the environment.
In this scenario, the Front End Server does not route network traffic to Microsoft Lync 2010 Mobile clients. For example, a Lync 2010 Mobile user cannot receive instant messages from federated users or from other Lync 2010 Mobile users.
This issue occurs because the Front End Server incorrectly tries to use the PSTN IP address to route the network traffic.
To resolve this issue, install the following cumulative update:
2675053 Description of the cumulative update for Lync Server 2010, Mobility Service: February 2012