Silence suppression, I thought. But looking over the configuration (snippet below):
dial-peer voice 100 voip
voice-class codec 1
session protocol sipv2
session target ipv4:172.16.0.220
session transport udp
All looked well with the world - VAD was turned off. ........... Well actually NO. I found out last night about the dreaded hidden default dial-peer which is used to match when other dial-peers don't - and guess what - this hidden, default dial-peer has vad enabled by default. The default dial-peer was being selected when Agent call legs and a outbound calls were being made from the IP side to the PSTN trunk.
incoming called-number .
Added to the voip dial-peer above fixes this problem by catching all the VoIP to PSTN dialled numbers in the "." - Job done. Watch out for this little gem...
I also discovered that the CIsco 2811 operating as a PSTN gateway with VAD enabled stops sending RTP packets when I muted my handset, i.e. no RTP packets during silence. Now cool this saves bandwidth - but SUCKS!!! What's a dynamic (de-)jitter buffer going to make of that? Not to mention a NAT firewall?