SIP response codes
SIP 503 Service Unavailable
Service Unavailable: the server cannot handle the call right now. Often the most common error on trunks.
- Name
- Service Unavailable
- Class
- 5xx · Server error
- Defined in
- RFC 3261
When it happens
- All channels on the trunk are busy (concurrent call limit reached).
- The trunk is not registered, or the provider is down.
- The PBX has no route that is currently up for this number.
- Overload or maintenance; a Retry-After header may say when to try again.
What to check
- Check the trunk registration and the concurrent call limit.
- Look at the PBX route list: a failover route should take over when the main one returns 503.
What the response looks like
SIP/2.0 503 Service Unavailable
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK776asdhds
From: <sip:101@pbx.example>;tag=1928301774
To: <sip:+15551234567@pbx.example>;tag=a6c85cf
Call-ID: a84b4c76e66710@203.0.113.10
CSeq: 2 INVITE
Reason: Q.850;cause=41;text="Temporary failure"
Content-Length: 0