When it happens
- The next hop is unreachable or slow.
- DNS resolution of the destination failed or timed out.
What to check
- Check connectivity and DNS from the server that answered towards the next hop.
What the response looks like
SIP/2.0 504 Server Time-out
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=102;text="Recovery on timer expiry"
Content-Length: 0