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 easy to understand and logical.