Confirm that your SonicWALL is set up correctly, especially Consistent NAT:
Confirm that in Asterisk SIP Settings, External Address and Local Networks are correctly set. If you change these, you must restart (not just reload) Asterisk.
Confirm that the extension has NAT Mode set to Yes, and that any NAT related settings in the client are turned off.
If you still have trouble, at the Asterisk command prompt, type
sip set debug peer xxx
where xxx is the extension number. Make a failing test call, paste the relevant section of the Asterisk log at https://pastebin.freepbx.org and post the link here. Also, report whether the desk phone user can hear the softphone user, and vice-versa.