Locate the response family
- 1xx ProvisionalRequest is progressing
- 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.
Start with the request
A response code is not a complete root-cause report. Identify the request method and the call leg before interpreting it. A 200 to REGISTER does not mean an INVITE succeeded, and an initial authentication challenge can be normal.
Understand the classes
1xx responses describe provisional progress. 2xx indicate success for the relevant request. 3xx provide redirection. 4xx describe request failures, 5xx server failures and 6xx global failures under SIP semantics. Each specific code has its own meaning and operational context.
Follow intermediaries
A PBX can terminate one leg and create another. An upstream failure may be translated before the handset sees it. Keep a bounded sequence from both sides where available, including timestamps and sanitized identifiers.
Use the reference practically
Choose the observed code below for its meaning, diagnostic checks and common misinterpretations. The entries distinguish normal progress from failures and explain what the code cannot establish by itself. For a user-facing symptom without a clear response, start in troubleshooting.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.