SIP response code

A SIP response code is a three-digit number a phone system sends back to say what happened to a call. SIP stands for Session Initiation Protocol, the signaling language behind most modern voice calls. The codes work much like the status numbers you see on the web — a quick, standard way to report success, redirection, a client mistake, or a server failure without writing a sentence.

The codes are grouped by their first digit. The 1xx range means the call is in progress, like 180 Ringing. The 2xx range means success, with 200 OK signaling that the call was answered. The 3xx range means redirection. The 4xx range means a problem on the requesting side, such as 404 Not Found or 486 Busy Here. The 5xx range means a server problem, with sip 503 being a common one. The 6xx range means a global rejection like 603 Decline.

Knowing these ranges makes debugging fast. If your sip calls fail and you grab a sip trace, the response code in the trace tells you immediately whether the fault is your request, the destination being busy, or your carrier having a bad moment. A wave of 503s usually points to congestion or an overloaded server rather than anything wrong with your specific call.

Keep in mind the response code is about signaling, while the reason the channel actually closed is reported separately as a hangup cause. The two usually agree, but when they differ it is a strong hint about where the problem lives. Learning to read response codes is one of the most useful skills for anyone running a dialer against real carriers.