At a low level, webrtc interoperates with VoIP. But because webrtc doesn't mandate a signalling protocol and VoIP uses SIP, generally you need a browser to run whatever proprietary signalling the site decided to implement in JS.
Anything can function as the signaling layer since all you need is to exchange pieces of text. I think the point was that most sites choose to implement it using JS and browser API's like ajax, websocket or SSE.
