On my iPhone X I am having the following issue when making a phone call from the native iPhone phone app.
I am unable to hang up. The red hang up button is visible but does not activate when pressed. I have to wait until the other party hangs up or reset my phone in order to end the call.
I am unable to switch apps when in the phone app. When I swipe up during a phone call, the other running apps are visible but if I try to switch apps the screen goes blank and it is not possible to regain focus i.e. the screen remains blank until the call ends.
I have found similar reported issues with regards to the red hang up button not being visible (the green button is visible instead), but I have not found anyone having the same issues that I am having.
I think you should get this checked at an Apple Store. I know someone who had a somewhat similar problem. He coudn't end the call sometimes. So he had to lock the phone, unlock it and only the red button would start working.
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