Just wanted to post my experience with a Sonicwall TZ-210 and our Asterisknow/Freepbx PBX. (SIP DID)
I was having issues with out bound calls being dropped after 15 minutes.
After trying everything I could find/think of. I decided to try changing the TCP timeout [Under Firewall -> Access Rules -> customer PBX rules/forwards. This value can be changed per firewall rules, so I changed it to 120mins[15 min was default]. I am not sure why this worked, but it did. Just wanted to share in case this helps someone else.