My iPhone X screen goes dark during calls ! Pressing power button twice wakes the screen . Why ?

The screen will go black when you hold the phone to your ear. It should turn back on when you move the phone away from your face. If it doesn't you have the phone in a case or a screen protector that is obscuring the proximity sensor that turns off the screen. Try removing your case; if that is the problem get a case designed for the iPhone X.

iPhones has a proximity sensor so when you put it up to your ear it turn off the screen to save power

My iPhone X goes black while on phone call and then I am unable to disconnect from call . Also I am unable to turn off I phone.