Interesting, I thought that notification thing only happened when the phone was unlocked.
Actually, did you disable your lock screen so you don’t have to enter a code to unlock your phone when you turn it on? I suppose that makes you never end up in a locked state and therefore it will always do it the notification way. Normally if your screen is off your phone is locked and thus you will get the full screen incoming call screen. I didn’t think of the lock screen being disabled possibly changing the phone’s behaviour.
Sorry for asking so many questions, trying to understand things better myself so I can help others better in the future 
No, it happens even when I haven’t the used the phone for a long time and the screen is locked. Once I have taken the call and finished, I then have to sign in again – I think it’s better to have a password to unlock, in case the phone were stolen.
Now I realise what I was doing “wrong”, but no one tells you these things, and as I have no children (or grandchildren), I’m on my own, so to speak.
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Admittedly this will be kind of a random shot, but …
Are there any Apps shown for you in Settings - Apps and notifications - Advanced - Special app access - Display over other apps?
One of them should be Phone, and with three-dot-menu - Show system another one should be Phone Services. I don’t know which one is responsible for the normal call user interface you don’t get currently.
Other Apps with this special permission could perhaps interfere, though, most probable would be Apps constantly modifying the display like blue light filter Apps or something like that. It might be worth a try to temporarily deactivate the other Apps with this special permission one by one to find out.
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Waiting till I get another call to see if it works. I also had power button ends calls off. But after opening phone app and going to settings within it, under calling accounts, there is a setting receive incoming calls. That was set to off on my phone.
