I've been having this issue for a while now and even the newest iOS didn't seem to solve it:
The problem is that sometimes I just won't receive calls and/or messages.
I figured it happens completely random, but in most cases it happens when the screen is locked - that means, even if I am passively using phone - listening to music and if the screen is locked, I'd sometimes get missed messages/calls - It doesn't ring, I don't get any alerts until after I wake up my phone. Then I get the notification of a missed call or that of a message that was delivered x minutes ago.
'Do not disturb' is off, I've also talked to people who couldn't reach me because of this problem and they said the phone was just ringing until the call was eventually dropped.
I've sent the list of missed calls to my carrier in case the problem might be on their side, but they've been unable to find anything out of the ordinary. So I guess the problem must be with the device itself or with the software.
Resetting network settings also doesn't seem to help at all.
Notification preferences in Settings → Notifications → Phone are all good. See comment under answer below: https://apple.stackexchange.com/a/379272/313842
I really like this phone, but if it doesn't serve it's main purpose, what can I do with it?