WebRTC. Specifically Google's libWebRTC for Android/iOS mobile platforms [1]. All the major web browsers (including Safari 11+) support WebRTC. For the server, I am interested in trying Jangouts (Janus Hangouts) [2].

  [1] https://webrtc.org/native-code/
  [2] https://github.com/jangouts/jangouts

Update: Forget to add the reference.