SIP is the Session Initiation Protocol. In IP and traditional telephony there are two different phases of a voice call.
The first phase is "call setup," and includes all of the details needed to get two telephones talking. Once the call has been setup, the phones enter a "data transfer" phase of the call using an entirely different family of protocols to actually move the voice packets between the two phones.
The SIP Monitor sends a Register, Options, Invite and Cancel messages.