WebRTC
WebRTC (Web Real-Time Communication) is an open standard built into modern browsers. It lets a web page capture a microphone or camera and exchange live audio and video with another party without a plug-in or separate app.
Also called: Web Real-Time Communication, browser calling
It is what makes "click to talk" possible on a website: the browser asks for microphone permission, negotiates a connection, and streams audio directly. Video calls in a browser tab, in-app voice chat and browser-based softphones all use it.
WebRTC handles the media and its encryption; something else has to handle signaling (often SIP or a WebSocket) and bridging to the phone network if the far end is a normal phone. For a voice agent, WebRTC is the channel by which a visitor talks to it from a web page rather than by dialing a number.