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Silence is there in between sentences on a call from an IP phone to the PSTN. VAD appears to be disabled


Core Issue

The call from an IP phone matches the default VoIP peer inbound to the gateway. The default dial peer has Voice Activity Detection (VAD) enabled.The absolute silence symptom is noticeable between speech.

These are characteristics:

  • The customer is running VoIP with VG200 gateways (12.2[7b]) and Cisco CallManager 3.22b.    
  • When establishing a call from VoIP to external phones, there is absolute silence between speech.       
  • When the user starts to speak, the first part of the first word is hard to understand. This behavior is similar to the behavior when VAD is enabled.       
  • When establishing a call from the external phone to VoIP, this behavior is not experienced and the voice is clearly understandable.     
  • Issuing the show call active voice command shows that VAD is disabled for a call from the Public Switched Telephone Network (PSTN) to the IP phone, but enabled for a call from an IP phone to the PSTN.

The network administrator has disabled comfort noise on the voice port. In addition, since there is no matching inbound VoIP dial peer, the default is used. Therefore, VAD is enabled in the outbound direction. The result is absolute silence between talk spurts for calls established from the IP phone to the PSTN, but not in the reverse direction.

The problem does not arise in the reverse direction because there is a match on the outbound VoIP dial peer, which is needed to route the call.


Ensure that the dial peer is defined. In addition, make sure it matches the call inbound on the VoIP side of the gateway. Issuing the incoming called-number command under a dummy VoIP dial peer ensures that all calls match an inbound  VoIP dial peer.

VAD can then be turned off on this dummy dial peer by issuing the no vad command.

For more information on matching inbound dial peers, refer to Understanding Inbound and Outbound Dial Peers Matching on IOS Platforms.

For more sample voice quality symptoms, refer to Recognizing and Categorizing Symptoms of Voice Quality Problems.

Also refer Cisco    CallManager Service Parameters for some relevant information.

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