A SIP call session between two phones is established as follows:
- The calling phone sends out an INVITE.
- The called phone sends an information response 100 – Trying – back.
- When the called phone starts ringing a response 180 – Ringing – is sent back.
- When the caller picks up the phone, the called phone sends a response 200 – OK.
- The calling phone responds with ACK – acknowledgement.
- Now the actual conversation is transmitted as data via RTP.
- When the person calling hangs up, a BYE request is sent to the calling phone.
- The calling phone responds with a 200 – OK.
It’s as simple as that! The SIP protocol is logical and very easy to understand.