XMPP too. It can do video calling: https://xmpp.org/about/technology-overview.html

Sad that Google + Facebook pulled out of federation a long time ago. We could have had bring-your-own-client cross-network all of this stuff. We still can, but realistically we won't.

WebRTC defines how the browser supports audio/video/chat endpoints. A practical video calling system needs servers to manage firewall traversal, a directory of users, and ways to admin the system.

Who's going to provide that at no cost?