Locate the response family
- 1xx ProvisionalRequest is progressing100 belongs here
- 2xx SuccessRequest succeeded
- 3xx RedirectionAnother destination
- 4xx Request failureInterpret this request
- 5xx Server failureServer could not fulfil
- 6xx Global failureFailure across destinations
Read the status with its request method and call leg. For example, 200 after REGISTER confirms a binding; it does not prove an INVITE succeeded.
What this response means
The next SIP hop has begun processing the request. This provisional response is normal progress, not proof that a phone is ringing.
Read it in context
A proxy or PBX can send 100 before contacting the final destination. If the call stops here, follow downstream processing and the next response rather than troubleshooting handset ringtone settings.
Administrator checks
Identify which system sent the response and whether it originated a downstream INVITE. Check routing, destination lookup and subsequent timeout or rejection. Preserve the full exchange: the useful failure often appears later on another leg.
Next action and common mistake
Do not count 100 as an answered or successfully delivered call. After a repair, verify actual alerting, final answer, acknowledgement and audio. A locally generated progress response can coexist with an unreachable carrier.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.