When you answer an incoming call from the ASUS Cover , your device would automatically turn off the screen to prevent accidental taps. So once you finished the call, you need to press the power button first, then operate by the following screen:
Tap and just swipe the Call Icon to the left to hang up and stop the phone app.
Note: If you use a non-official cover accessory, some functions may not work properly. Check out the official ASUS website to see the latest ASUS official cover accessory.