https://hpbn.co/
Then for a practical example, try this Codelab on Real time communication with WebRTC:
https://codelabs.developers.google.com/codelabs/webrtc-web/#...
Once you are ready to build your own prototype. You can check out existing implementations from PeerJS, SimpleWebRTC, SIP.js, and Twilio.
Although it is relatively easy to invent your own. Dedicated third party cloud providers such as Twilio are quite mature. Best of luck!
http://peerjs.com/
https://sipjs.com/
https://simplewebrtc.com/
https://github.com/coturn/rfc5766-turn-server/
https://github.com/otalk/restund
https://talky.io/
https://xirsys.com/
https://www.twilio.com/webrtc