SIP Response Codes — What Every Code Means

Why Response Codes Matter

Every SIP transaction ends with a response code. As a NOC engineer, these three-digit numbers are your first diagnostic signal — they tell you what happened before you ever open a packet capture. Knowing response codes by heart is like a doctor knowing vital sign ranges: it immediately narrows the diagnosis.

SIP borrowed its response code scheme from HTTP, organized into six classes. But unlike HTTP, SIP has provisional responses that arrive during transaction processing, not just at the end. This distinction is critical for understanding call setup timing.

1xx — Provisional Responses

Provisional responses indicate the request is being processed but hasn't reached a final state yet. They keep the transaction alive and prevent timeout.

100 Trying

The first hop received your INVITE and is working on it. This is hop-by-hop — each proxy sends 100 Trying back to its upstream immediately. It exists solely to stop Timer A (INVITE retransmission) from firing. If you don't see a 100 Trying, the request probably never reached the next hop.

180 Ringing

The called endpoint is alerting (ringing). This is end-to-end — it means the INVITE reached a real device and that device is presenting the call to a human. In NOC operations, seeing 180 Ringing but no 200 OK means the call reached the destination but wasn't answered. The distinction between "no 180" and "180 but no 200" is crucial: the first is a routing/reachability problem, the second is a user behavior issue.

183 Session Progress

This is arguably the most important provisional response for VoIP. 183 with an SDP body establishes early media — a media path before the call is answered. This is how you hear ringback tones, announcements ("the number you have dialed..."), and IVR prompts from the far end before answer. Without early media, callers would hear silence until the call connects.

💡 NOC Tip: customers report "no ringback tone," check whether 183 Session Progress with SDP is being received. Some configurations strip early media, causing silence during ring phase. slug: "sip-response-codes"

2xx — Success

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