When it happens
- A typo or a missing digit in the number.
- The dial plan has no route for this prefix.
- The extension is not configured, or the call reached the wrong server.
- The number was sent in a format the server does not recognise (with or without +, country code or trunk prefix).
What to check
- Compare the Request-URI with what the server expects.
- Check the dial plan and number normalisation on the PBX.
What the response looks like
SIP/2.0 404 Not Found
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=1;text="Unallocated (unassigned) number"
Content-Length: 0