When it happens
- The callee’s device received the call and is alerting the user.
What to check
- If the caller hears silence instead of ringback, the phone expects early media (183) and gets none, or local ringback is disabled.
What the response looks like
SIP/2.0 180 Ringing
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