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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 ****
Enabling “Consistent NAT” can also resolve a call control drop issue. Looks like Sonicwall would change the port used for call control roughly 10 minutes into the call.
After enabling Consistent NAT calls should stay established until either the called or calling party disconnects the call.