Assume that a Microsoft Lync 2010 client supports Secure Real-time Transport Protocol (SRTP), but does not support Scale-SRTP. When a dial-out call is sent to the Lync 2010 client, a one-way audio media flow is established between the Lync 2010 client and the Audio Video Multipoint Conferencing unit (AVMCU).
This issue occurs because the AVMCU incorrectly applies the crypto-scale SRTP key to the audio channel. However, the Lync 2010 client uses the non-scale MKI key. Therefore, the client cannot decode media from the conference.
To resolve this issue, install the following cumulative update:
2514978 Description of the cumulative update for Lync Server 2010, Conferencing Server: November 2011
For more information about the method to join C3P addUser dial-out conference, visit the following Microsoft website: