Has anyone tried NextCloud Talk? https://nextcloud.com/talk/
Apparently, a replacement for their use of the WebRTC server Spreed. For future releases they are planning server-to-server federation for calling, and an XMPP integration, and they have talked to the Matrix team about possible integration with the Matrix protocol too:
http://karlitschek.de/2018/01/nextcloud-talk-is-here/
