When it happens
- Two PBXs or proxies route the same number to each other.
What to check
- Check the routes for this prefix on every hop; make sure the trunk does not route back to its source.
What the response looks like
SIP/2.0 482 Loop Detected
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=25;text="Exchange routing error"
Content-Length: 0