The request has a problem on the sender’s side or cannot be completed at this server. Retrying the same request will not help.
What the response looks like
SIP/2.0 416 Unsupported URI Scheme
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=127;text="Interworking, unspecified"
Content-Length: 0