When you try to make or receive Lync Server-based calls through the public switched telephone network (PSTN), the calls are unresponsive. Additionally, Lync VoIP calls that are routed to and from the PSTN experience poor audio quality.
This issue occurs when the collocated Lync Server Mediation Server is configured to use only a single network adapter.
Note For all Lync Server roles, one network adapter is required, but two network adapters are recommended. Each network adapter should have a bandwidth of 1 gigabit per second (Gbps) or greater.
For more information about Lync Server hardware requirements, see the following TechNet article:
To resolve this issue, add a second network adapter to the Lync Server front-end server that hosts the Mediation Server role. This approximately doubles the network throughput to and from the Lync Server front-end server that hosts the Mediation Server role.
An additional network adapter can be added to the Windows Server-based computer as a teamed network adapter solution. Teamed network adapters let you use dual port configurations that effectively double the throughput of network traffic that the Lync Server front-end server can send and receive. This generates more available network bandwidth for the additional SIP signaling and audio media traffic that's created by the collocated Lync Server Mediation Server role.
Note Teamed network adapter solutions are supported by the server or hardware vendor. For more information, see the "More Information" section.
For more information about these issues, see the following TechNet articles: