When it happens
- A BYE or re-INVITE for a call that already ended on the other side.
- The call state was lost after a server restart or failover.
- NAT changed the source port mid-call and the request no longer matches the dialog.
What to check
- Look at what happened earlier in the dialog: often the far end already sent a BYE that you did not receive.
What the response looks like
SIP/2.0 481 Call/Transaction Does Not Exist
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=41;text="Temporary failure"
Content-Length: 0