To disable SIP Transformations, perform the following:
1. Login to the SonicWall web interface.
2. On the left columns, locate the VoIP tab. Depending on the software version you're using, your screen may look slightly different.
3. Enable “Consistent NAT”
4. Make sure that ALL of the options are unchecked under SIP Transformation.
5. Click “Apply”.
6. Reboot your Siteserver VoIP handsets.