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When you call Public Switched Telephone Network (PSTN) users through Audio Video Multipoint Conferencing Unit (AVMCU) in a Microsoft Office Communications Server 2007 R2 conference, the comfort noise packets are not added for the PSTN users. This causes voices to sound stretched.
To resolve this issue, apply the following update: For more information, click the following article number to view the article in the Microsoft Knowledge Base:
971793 Description of the update package for Office Communications Server 2007 R2, Audio/Video Conferencing Server: July 2009
This update improves voice quality and reduces voice stretching instances for PSTN users.