SonicOS has a feature called SIP Transformations that may cause issues with your VOIP end points. Providers will often ask you to disable this setting if you are experiencing call control or quality issues.
To disable SIP Transformations:
1. Log into the web interface on the SonicWall.
2. On the left, find the VOIP tab. Depending on the version of SonicOS your screen may appear slightly different.
3. Make sure that ALL of the options are unchecked on this page.
* Prior to clicking on apply, make sure that there are no active calls. They will be disconnected or audio will be lost.
4. Click on “Apply”.
5. Reboot your VOIP endpoints.
**** UPDATE – TZ170 12/8/2011****
I recently needed to have someone enable “Consistent NAT” to resolve a call control drop issue. Our soft switch would send a SIP UPDATE packet to the IPT at roughly 10 minutes into the call. After no response from the IPT behind the TZ170 the call would drop. After further investigation I found that the port used for call control was changed by the Sonicwall at about 9 minutes 30 seconds.
After enabling Consistent NAT calls would stay established until either the called or calling party disconnected the call.