Ok, I finally solved my issue after hours on the phone with support for firewall and VOIP provider. Here is my setup and the solution. VOIP Provider - ViaTalk, Firewall - SonicWall 205, Axon PBX Server and Grandstream GXP1405 IP Phone. The following settings were changed. 1. Use non-Asterisk based server from VOIP provider. 2. Have SonicWall VOIP options turned off and the following ports forwarded to the PBX - 5060, 5070, 8000-8020. 3. Ensure the PBX is registering the external IP address when starting. Because the SonicWall is a 'reflexive' NAT, STUN will not work. In addition, SonicWall does not support UPNP. So if you have a dynamic external IP Address, you will need to manually put that address into the Static IP option in Axon and update everytime the DHCP server gives you a new address. I suppose you could use a dynamic DNS address if axon supports that. Now I have audio both ways on incoming calls.