SIP

SIP is the IETF standard protocol for initiating, maintaining, and terminating real-time sessions including voice, video, and messaging.

Session Initiation Protocol (SIP) functions as the signaling backbone for modern IP communications. Defined by RFC 3261, it handles the setup and teardown of media streams by negotiating parameters through the Session Description Protocol (SDP). It operates independently of the underlying transport layer (typically UDP or TCP on port 5060) and manages critical functions like user registration, call forwarding, and multi-party conferencing. Whether it is a Zoom meeting or a corporate VoIP handset, SIP ensures the endpoints agree on codecs and encryption before the data starts flowing.

https://www.ietf.org/rfc/rfc3261.txt