Explanation: The INVITE message is used to initiate a new SIP session by sending an invitation to the called party. It includes information about the caller's capabilities, desired media types, and other relevant details necessary for establishing a successful connection. Upon receiving this message, the called party can respond with a 180 Ringing response to indicate that they are alerting the user or a 200 OK response if they accept the call. This process allows for the negotiation of session parameters before the actual communication begins.