Three SIP Response Codes That Mislead You Most Often
Standalone chapter: 403, 480, and 503 get read backwards during trunk troubleshooting. Here is what they usually mean.
SIP response codes were designed for machines, not for a person in the middle of an incident. These three get misread the most.
403 Forbidden
Usually read as an authentication problem. On an enterprise trunk it more often means the calling number is not permitted by policy on the far side, and the credentials are fine.
480 Temporarily Unavailable
The word “temporarily” makes people wait. In many configurations this is sent when no route matches at all, and waiting changes nothing.
503 Service Unavailable
The most honest of the three: the far side really cannot take the call. Still worth checking whether the 503 came from your own CUBE running out of resources rather than from the destination network.
What to do with this
Check who sent the response before deciding what it means. The same code from CUBE and from a carrier sends you down two completely different investigations.