Identify the method, transaction and rejecting hop
Capture one complete failed exchange with the request method, Request-URI, Call-ID, CSeq method and number, top Via branch, Route set, source and destination. Establish whether Asterisk received 501 Not Implemented from a phone, proxy or provider, or generated it while acting as the server. Asterisk can terminate one SIP leg and create another, so correlate each PJSIP channel separately.
Preserve the user action that caused the request: transfer can create REFER, messaging can create MESSAGE, a feature can use INFO, and event state can use SUBSCRIBE, NOTIFY or PUBLISH. A 501 on one of those transactions is not automatically an INVITE or media failure.
Prove whether 405, 420 or 501 fits
A 405 response means the responder understands the method but disallows it for that Request-URI and supplies Allow. A 501 says the responder lacks functionality required to fulfil the request. A 420 instead rejects an unsupported required option tag and can identify it in Unsupported. Compare the actual final response and headers before changing configuration.
Do not convert 501 into a permissions diagnosis solely because a different URI accepts the same method. Conversely, do not install or enable a handler when the responder's Allow evidence shows the method is understood but barred for this resource. Preserve customized reason phrases as text, not standardized root causes.
Inspect Asterisk's loaded feature owner
Use a narrow pjsip set logger on reproduction and correlate the exchange with the Asterisk log. Then use CLI help and module information to identify the application or resource module expected to own that method and feature on the running branch. Confirm the loaded endpoint and inbound context without editing generated FreePBX files or enabling unrelated modules. Disable protocol logging when the capture ends.
Method recognition, endpoint identification, application policy and downstream feature support are separate stages. An endpoint that matches correctly can still reach no handler for a method; a loaded module does not prove the request URI, event package, content or permissions are valid.
Diagnose a received 501 on the outbound leg
When Asterisk receives 501, preserve the selected endpoint, contact, outbound proxy and actual next hop. Determine whether the rejecting system is an intermediate proxy or the intended target and compare its current interoperability documentation for that method. Do not assume that a provider supports REFER, MESSAGE, INFO or PUBLISH because it supports ordinary voice INVITEs.
Avoid method substitution as a generic repair. An attended-transfer fallback, out-of-dialog messaging path or application-specific retry is valid only when the controlling feature specification and peer documentation define it. Test the exact user workflow after a bounded change and retain a control call that does not invoke the method.
Keep the original SIP code beside translated causes
Asterisk's hangup table groups unlisted SIP 5xx responses into a broader switching-congestion cause. DIALSTATUS, HANGUPCAUSE and per-channel technology causes can help explain dialplan behavior, but they are not substitutes for the captured 501 and can lose the method and responder identity. Use HANGUPCAUSE_KEYS() and per-channel inspection only when a call created relevant channels.
A 501 for an in-dialog feature request may not hang up the established call at all. Do not infer dialog termination, capacity exhaustion, carrier outage or retry safety from a generic cause label. Preserve the request's own extension-defined state and final outcome.
Verify one supported outcome
Correct only the evidenced ownership, routing, module, feature policy or peer-compatibility issue. Repeat the same method, URI, dialog state and user action; confirm the new final response and application result, then verify the surrounding call or subscription state and a nearby control. If the peer truly does not implement the function, document the supported workflow rather than hiding 501 with an unproven rewrite.
A successful bounded test proves only that method and path. It does not establish every endpoint, provider route, transfer mode, message type, dialog state or software version. No PBX, endpoint, proxy, provider, method, module, application, transaction, feature or live call was tested for this guide.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- IETF RFC 3261: method, 405, 420 and 501 behavior ↗
- IETF RFC 4475: unknown-method SIP torture tests ↗
- Asterisk PJSIP troubleshooting guide ↗
- Asterisk hangup cause mappings ↗
- Asterisk per-channel hangup cause inspection ↗
- Asterisk CLI syntax and help ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.