Facetime doesn't close on iPhone X, can't restart the phone

Hi Apple,

Every tenth time Facetime stops working properly on my iPhone X iOS 11.3.x (can't follow the number of the patches , anyway the latest available one in May 2018)

It doesn't matter, because same problem persisted before this latest update neither.

Symptom: After one of the partner hangs up a call, the screen shows the 'Call ended' text and all of the buttons (among the red one in the bottom center) turns unavailable.

But that's all, the app doesn't actually close.

I can't force close the app neither, I can close every other app but this one doesn't have the little red symbol on its top left corner.

If I try to make an other FaceTime call, it just shows up on the screen like it is waiting for the previous call to be finished, but never actually goes through. When I hang up this new call it goes to the list, so two calls are 'ending' but never go away.

Wanted to take a screen video, but as soon as I'm trying to swap back to FaceTime, Screen capturing is asking if I want to stop recording (Options are Stop' and 'Cancel')

An other thing, and makes it even more annoying, I can't restart the phone using the Power and Volume keys, because it only brings the frozen Facetime app windows to foreground.

The only way how I can make to phone works again to go into the Settings, find 'Shut Down' under General and proceed.

Anybody else has the same issue?

Solution?