When it happens
- The callee picked up, or a PBX, IVR or voicemail answered.
What to check
- If the 200 OK is repeated several times and the call drops after about 32 seconds, the ACK is not getting through: usually NAT or a wrong Contact header.
What the response looks like
SIP/2.0 200 OK
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
Content-Length: 0