We are currently experiencing call disconnect after 6 minutes, when calling through a Sonicwell firewall vpn connection.I went forward and made a few changes on my Sonicwall between both sites. This seems to have resolved the issue. There are VOIP settings on the Sonicwall’s which are meant to make the VOIP more efficient. After reading through their documentation I was able to find that these settings are really meant for WAN to LAN VOIP not VPN to VPN.
Under voip ->settings: Enable consistant NAT, and disable all h.323 options.