title: Override default call interface
body: <p>I am aware that doing this is possible in Android (<a href="https://stackoverflow.com/questions/40888058/how-to-override-default-call-screen">here is a related question</a>). However, currently I am very puzzled about whether it is even possible to do this on Windows Phone.</p>
  
  <p>Having had a look at the APIs, it appears that the call GUI has to be in the foreground (so launching it as a background task will not work). I've also considered if a popup can be created on top of the call interface.</p>
  
  <p>This is a request (and one of the most important ones) of my client so while I strongly disagree with the idea of overriding the default call interface, this is more or less the requirement.</p>
  
  <p>Thank you very much in advance!</p>