Voice Activity Detection (VAD) appears to be disabled in the gateway configuration but is active on the call. The call from the IP phone to the Public Switched Telephone Network (PSTN) is matching the default VoIP peer inbound to the VoIP side of the gateway. The default dial peer has VAD enabled.
Ensure that the call matches the correct VoIP or Plain Old Telephone Service (POTS) dial peer as it enters the Cisco IOS voice gateway.
Inbound VoIP dial peers are usually not configured because they are not required to route the call. However, VAD is configured on the VoIP dial peer. If there is no matching inbound dial peer, the default dial peer (dial peer 0) is used and has VAD enabled.
Eliminate hissing by issuing the incoming called-number <number_dialed> command on the dial peer to allow for the matching incoming dial pattern. The exact pattern is not required. A period may be used to represent any number. Ensure the no vad command is also issued in this dial peer.
This an example configuration:
dial-peer voice 100 voip
incoming called-number .
!--- This matches all incoming numbers.
session target ipv4:10.10.1.1
The incoming called-number command is recommended. It matches on the called number (Dialed Number Identification Service (DNIS)) of the incoming call that is always included.