SIP
SIP (Session Initiation Protocol) is the standard signaling protocol used to set up, modify and end voice and video calls over IP networks. A SIP trunk is a SIP connection from a phone system to a carrier.
Also called: Session Initiation Protocol, SIP trunk, SIP trunking
SIP handles the conversation about the call, who is calling whom, whether they answered, when it ended, while the audio itself travels separately, usually as RTP packets. Most business VoIP, from desk phones to cloud contact centers, speaks SIP.
A SIP trunk replaces physical phone lines: the business’s phone system connects to a carrier over the internet and the carrier bridges to the PSTN. Trunks are sold by concurrent channel, which is what "how many calls at once" usually comes down to.
SIP is also how an AI voice service is typically connected to an existing phone system, as a SIP endpoint the system forwards or transfers calls to.