When it happens
- Sent by every proxy or PBX within milliseconds of the INVITE.
What to check
- If 100 Trying is the last thing you see, the call is stuck further along the route: look at the next hop.
What the response looks like
SIP/2.0 100 Trying
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK776asdhds
From: <sip:101@pbx.example>;tag=1928301774
To: <sip:+15551234567@pbx.example>
Call-ID: a84b4c76e66710@203.0.113.10
CSeq: 2 INVITE
Content-Length: 0