matrix.org/partials/frontpage/features/voip.md

VoIP

With the advent of WebRTC, developers gained the ability to exchange high quality voice and video calls – but no standard way to actually route the calls.

Matrix is the missing signalling layer for WebRTC. If you are building VoIP into your app, or want to expose your existing VoIP app to a wider audience, building on Matrix’s SDKs and bridges should be a no-brainer.