401 - Unauthorized - username/password invalid
403 - Forbidden - means username/password is incorrect or PBX rejected the call for some reason (should be seen in logs)
404 - Not Found - usually means that customer dials some incorrect number OR they're trying to send call to correct IP but incorrect port causing call to go to one of our PBXs. You should be able to see something like 'Call to incorrect number' or 'Extension not found in context' in PBX logs.
407 - Proxy authentication required - username/password is incorrect
488 - Not acceptable here - phone is registered, but codecs are incorrect, i.e. softphone has G711 Alaw enabled while server expects G729. This can happen if customer was complaining about bad call quality and you've switched them over to G729 but didn't take into account that softphones don't support G729 (except for Bria - whcih you have to pay for).
Please note 404 and 488 when troubleshooting any issues! These ones are important.