Consider the following scenario:
You have a Polycom CX 600 or Polycom CX 700 telephone that is running Lync 2010 Phone Edition.
You connect the telephone to a computer that is running Lync 2010.
You answer a Public Switched Telephone Network (PSTN) or a Private branch exchange (PBX) call from user A by using the Polycom telephone.
You receive another PSTN or PBX call from User B during the conversation with User A.
In this scenario, the call from user A is disconnected. Additionally, you are connected to user B without an indication or notification.
This issue occurs because the PBX or PSTN calls contain the same header information. Therefore, the Lync 2010 client cannot distinguish the incoming calls and handles it as a personal transfer.
To resolve this issue, install the following cumulative update:
2571543 Description of the cumulative update package for Lync 2010: July 2011