I have a 7940 connected to CCM over SonicWall VPN tunnel. When i am making internal call all works, but when i am trying to call / or receive external calls i have no voice.
Please make sure the subnet range tha ip phone belongs to, it allowed over the tunnel, in addition to port 2000 (SCCP), 5060 (SIP), the VPN tunnel has to permit the RTP ports range "16384-32767"
If you can call IP Phone to IP Phone from your main site to your remote site then I would say it is probably your voice gateway? Does your voice gateway have a route to the remote VPN subnet?