SIP
The protocol that sets up and tears down calls over IP — the signalling, not the audio.
The Session Initiation Protocol was specified in RFC 3261 and resembles HTTP in construction: text-based messages with methods such as INVITE, ACK and BYE. Addresses look like email addresses. The audio itself travels separately over RTP; SIP only negotiates where and how.
The protocol underpins corporate PBXs, SIP trunks to operators, and IMS, which carries voice in 4G and 5G. The recurring headache is NAT: SIP puts IP addresses inside the messages, which breaks as soon as a firewall translates them, and has spawned a whole family of helper protocols.