When it happens
- Routing loop between servers.
- A very long chain of proxies.
What to check
- Find the loop in the routing tables; check Max-Forwards in the captured INVITEs to see how it drops at each hop.
What the response looks like
SIP/2.0 483 Too Many Hops
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