I am trying to achieve the same results that can be seen at 29:52 seconds Improving VoIP Applications with CallKit - WWDC 2016 . This video shows that after answering a call, the buttons move and change with animation, and the screen remains in its own user interface.
I built Speakerbox and tried the "Simulate an incoming call" function, but as soon as the call answer button answers the call (the same action as shown in the video), the main call screen disappears and the application screen is displayed with the active calls shown in the list ( which is different from what is shown in the WWDC video):
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