I had an unusual problem recently with certain calls going to the PSTN via a SIP provider. The call would connect but with no audio at either end.
I’ve seen this lots before and is often caused by NAT or a firewall blocking the audio stream but that wasn’t the cause this time.
The problem was caused my trunk only being setup to allow the ulaw codec (allow=ulaw on the trunk). What I think was happening was that my provider was accepting, and connecting, the call but then when it tried to hand the call off to it’s upstream provider, which only accepted alaw, it would fail.
So if you’re having problems with connected calls but no audio it might be worth enabling all of the codecs on the trunk to rule out any codec mismatch issues.
If that doesn’t help look at NAT or firewalling 🙂