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Consider the following scenario:

  • An Enterprise Voice-enabled Microsoft Lync 2010 user creates a conference.

  • Twenty users join the conference by using Public Switched Telephone Network (PSTN) telephones. The participants in the conference are not muted.

  • User A joins the conference by using a PSTN telephone.

In this scenario, the participants cannot hear user A. However, if one of the first 20 PSTN participants is muted, user A can be heard.


This issue occurs because the smart-mixing feature should not be enabled in the Personal Virtual Assistant (PVA) group. The smart-mixing feature accepts only 20 unique inputs in every audio mixer.

Note Audio Video Multipoint Conferencing Unit (AVMCU) uses RtpConferenceGroup groups to handle customized audio mixing. Users in the same group can hear one another. Lync 2010 client users are in the main group, and this group has the smart-mixing feature enabled. The PSTN users are in both the main group and the PVA group that is used to handle prompts.


To resolve this issue, install the following cumulative update:

2606429 Description of the cumulative update for Lync Server 2010, Conferencing Server: August 2011

More Information

For more information about Personal Virtual Assistant, visit the following Microsoft website:

General information about Personal Virtual Assistant
For more information about audio mixers, visit the following Microsoft website:

General information about audio mixers

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