Conferencing Attendant could not negotiate TLS and failed to establish a SIP connection. Reason=%1. Front End=%2. Cause: ...

Conferencing Attendant could not negotiate TLS and failed to establish a SIP connection.

Reason=%1. Front End=%2.
Cause: This issue may occur due to TLS certificates on the Microsoft Lync Server 2010 Front End and the Conferencing Attendant being expired, or not in sync.
Resolution:
Check and resolve certificate issues, mismatches between the Microsoft Lync Server 2010 Front End and Conferencing Attendant to restore connectivity.