Session Initiation Protocol (SIP) is used to set up peer-to-remote calls, peer-to-peer calls, and SIP trunk calls.
SIP is a signaling protocol widely used for initiating, maintaining, and terminating real-time communication sessions, including voice, video, and messaging applications. It facilitates communication between endpoints, whether they are directly connected or through a SIP trunk. Axis Communications' technical documentation on SIP and VoIP solutions confirms these uses.