Actually this is quite doable. Not as a native application however (though that itself IS possible using the Discord SDK, considering that there is a wonderful third-party UWP Discord application for Windows), but using the webapp-container with the user-agent string of the latest Chrome or Chromium release, will cause the webapp to request permission for Microphone and allow VOIP calling under Ubuntu Touch. It will not however, allow PTT (as PTT requires a native application).