Calls and messages won't come through when screen is locked iPhone SE

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?