SIP Response codes and their meaning
Here are the most common response codes generated by the Net-Net System and a possible reason why it has sent this response. Please note that these are the responses generated by the system and not by a next-hop server (where system is acting as a back-to-back user agent).
If the output of show sip invite or show sip reg has the same number of 4xx or 5xx counted as received by the client as sent by the server, then this document doesn't apply. The Net-Net system is simply proxying these through the SBC (IE: the error is generated by another SIP server).