503 5xx Server error RFC 3261
SIP 503 Service Unavailable
The server is temporarily overloaded or under maintenance. It may include Retry-After. Many PBXs also send 503 when no outbound route or trunk is available.
Typical Q.850 / ISDN cause: 41 (Temporary failure)
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Common causes
- No working outbound trunk or route (Asterisk: dial failed / congestion).
- Trunk channel limit reached, or carrier overloaded.
- Proxy has no healthy destination (dispatcher or drouting all down).
How to fix it
- Check trunk registration and status on your PBX.
- Check carrier capacity, channel limits and account status.
- In OpenSIPS or Kamailio, check the dispatcher or gateway probing state.
Seeing 503 on calls through a router or firewall? Check that SIP ALG is disabled. It causes a surprising number of SIP errors. Read why.
About 5xx responses
The server failed to handle an apparently valid request.