When it happens
- The carrier behind your provider rejected the call.
- An upstream trunk is down or misconfigured.
What to check
- Check the trunk status on the PBX; ask the provider which upstream route failed.
What the response looks like
SIP/2.0 502 Bad Gateway
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=38;text="Network out of order"
Content-Length: 0