title: How are the CXStartCallAction isVideo property, and native video button used in CallKit?
body: <p>I'm trying to figure out what happens when the user taps the video button in the native iOS call screen. I've gone through a few tutorials and looked at the code and it doesn't appear to be a delegate method or anything.</p>
  
  <p><code>CXStartCallAction</code> has a property: <code>var isVideo: Bool</code>, but I cannot seem to figure out how it is used.</p>
  
  <p>I've noticed that (in the Speakerbox example project) it changes the display from "Speakerbox Video" to "Speakerbox Audio", but I'm assuming it has other uses.</p>
  
  <p>The bigger question may be "what does the video button do?" Seems like it just opens the open, but not sure.</p>