Operations guide · SIP
SIP Troubleshooting
Causes, Checks & Fixes
A repeatable, evidence-led method for diagnosing signalling, routing and media failures.
Start with evidence. Use the complete signalling ladder, both media directions, synchronised timestamps and the exact affected Call-ID.
A disciplined method
- Define the boundaryRegistration, setup, progress, answer, media, mid-dialog request or teardown?
- Identify the transactionUse Via branch and CSeq; use Call-ID and tags for the dialog.
- Follow every hopCompare what entered and left each proxy or SBC.
- Separate SIP from RTPSuccessful signalling does not prove media delivery.
- Change one variableRetest and preserve a known-good trace for comparison.
Common failures
Registration fails
401 challenge is normal once; repeated 401/407 suggests credentials, realm, nonce or digest mismatch. 403 normally indicates policy rejection. Check REGISTER Request-URI, From/To, Contact, expiry and response.
No response / timeout
Confirm DNS and next hop, transport, destination port, firewall policy and source-IP authorisation. Retransmitted UDP requests show the sender is not receiving a response.
Call ends after about 30–32 seconds
A common cause is a missing ACK to 200 OK. Follow Via, Record-Route, Contact and NAT paths and confirm the ACK reaches the answering user agent.
One-way or no audio
Compare SDP connection addresses and media ports, then capture RTP in both directions. Check NAT, firewall, media anchoring, SRTP policy and whether private or unreachable addresses were advertised.
488 Not Acceptable Here
Compare offered and accepted codecs, payload mappings, media direction, DTMF, SRTP requirements and SDP syntax. Never diagnose 488 from the status text alone.
503 Service Unavailable
Check overload, maintenance, capacity, upstream failure and Retry-After. Protect alternate routes from retry storms and confirm whether the error is local or downstream.
How to read the trace
Verify Request-URI, next hop, Via branches, Max-Forwards, Route/Record-Route, Contact, From/To tags, Call-ID and CSeq. Check whether provisional and final responses return on the expected path and whether ACK or BYE follows correctly.
Media diagnosis
Compare both SDP offers and answers: connection address, audio port, transport profile, payload numbers, rtpmap, fmtp, direction attributes and crypto or DTLS information. Measure packet loss, jitter, sequence gaps and RTP source/destination independently in each direction.
Capture checklist
- Exact UTC time, calling and called numbers, Call-ID and route
- SIP ladder from both interconnection sides
- SDP offer/answer and RTP stream statistics
- DNS result, transport state, SBC policy and firewall decision
- Expected behaviour, observed behaviour and last known-good test