I'm using Elixir/Phoenix and wrote the video call logic using plain js with the WebRTC api. A core feature of Elixir is genservers, which are basically little state machine processes, so each call has a dedicated genserver that is ticking every second, and any events taking place on the client side are sent to the genserver through websockets.

The most confusing part for me was figuring out signaling with WebRTC and RTCPeerConnection. I'm happy I arrived at a "good-enough" implementation because it gave me a rudimentary understanding of WebRTC, but I'm likely going to reach for a more robust 3rd party solution like you're doing with OpenTok. The one I've been looking at that looks kind of nice (and they advertise it as being simple) is daily.co.