Hi, in short:
On the backend side you need two things:
- a turn server
- a server to handle signalling (XMPP, MQTT…)
On the client side you need:
- WebRTC
- a client for the signalling protocol you choosed
The signalling will not work when the callee is offline as it is not possible to have a permanent background connection on Android or iOS that the operating system will not kill. So your signalling server needs to send push messages via Google’s Firebase service to wake the callee up.
