Session Invitation

A session invitation consists of one INVITE request which is usually sent to a proxy. The proxy sends immediately a 100 Trying reply to stop retransmissionsi and forwards the request further.

All provisional responses generated by callee are sent back to the caller. See 180 Ringing response in the call flow. The response is generated when callee's phone starts ringing.

SIP Invite Transaction Call Flow

A 200 OK is generated once the callee picks up the phone and it is retransmitted by the callee's user agent until it receives an ACK from the caller. The session is established at this point.