SIP is a Voice over Internet Protocol (VoIP) service. It is an application-layer control protocol; a signaling protocol for Internet Telephony. In IP and traditional telephony, network engineers have always made a clear distinction between 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. In the world of VoIP, SIP is a call setup protocol that operates at the application layer.