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IP phones negotiate g729br8 with an MGCP enabled gateway. As a result, VAD cannot be disabled
Since G.729 Annex B provides silence suppression, it is an inherent property of g729br8 to not allow disabling vad.
The problem occurs when the Cisco CallManager (version 3.2(2c) spE) region is configured for calls to use the G729 codec. When an IP Phone (load P00303020204) makes a call to an Media Gateway Control Protocol (MGCP)-controlled gateway, it negotiates g729br8. If Voice Activity Detection (VAD) cannot be disabled, certain voice quality issues that would ordinarily require VAD to be disabled may not resolve.
This issue is seen whether the IP Phones and VG200 are in the same or different regions.
Upgrade Cisco CallManager to version 3.3(2.27).
This release adds this service parameter:
Strip G.729 Annex B (Silence Suppression) from capabilities
If this Cisco CallManager service parameter is set to True, Cisco CallManager does not advertise or negotiate annex b for G.729 codecs, unless that is the only matching codec (G.729b or G.729ab).
This issue is also documented in Cisco bug ID CSCdz70790.
Hissing (Listen to short or long hissing samples.)
Clipped voice (Listen to short or long clipped voice samples.)