When it happens
- The callee pressed Reject.
- A call-blocking rule or anti-spam filter refused the caller.
What to check
- If calls are declined automatically, check blacklists, anonymous-call rejection and spam filtering on the callee’s side.
What the response looks like
SIP/2.0 603 Decline
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=21;text="Call rejected"
Content-Length: 0