SonicWalls are one of the harder firewalls to deal with when it comes to VOIP. I have several cheat sheets I can send if interested, it gotten to the point that if the IT person or company has never done VOIP we will go around them with public IP's.

However we have chased issues with everything being public and had these issues as well. In those cases it is always the carrier blocking something or in our case the Comcast modems blocking or having a setting activated that should not be.

Make sure you have good up to date firmware on the IP cards and the correct phone firmware. I have seen a bunch of LIPU's that will work flawless with older firmware on the 5xxx series phones but crap out with the latest and greatest.