My setup is as follows:
CCM5.1 - IPIPGW - PSTN softswitch
I can establish call connections from either side but once the called side picks the phone, no voice is heard. the connection is just quiet and nothing is heard. find my configs and debugs output attached.
Take a 'show voip rtp connections' while making the call. You should see a local side and a remote side. Then from the GW where you are taking that command perform a 'ping remote_IPAddress source local_IPAddress' with the IP addresses that you saw on the show voip rtp connections and see if you can reach them. Audio problems are almost all the times routing issues.