FP3+, sporadically can't answer

I have an FP3+ since July 2021. Some months (6?) ago, it started sporadically refusing to let me answer an incoming call. (Problem might have been there all along - I wasn’t taking that many calls on it before that.) That is, I can hear it ringing, but there’s nothing on screen, and if I pull down to check notifications, or show the list of running apps, or even open the phone app directly, nothing, no way to answer. Sometimes I’ve guessed who it was, call them back, and they confirm it was ringing but they didn’t get an answer. I changed service provider a month or so ago, so obviously new SIM card too, though the problem might have been their end, but same problem with new provider. Tried installing a different call-handling app - didn’t help so have switched back to the Android default. I found these two posts from people with a similar problem, but they don’t apply: Can't answer the phone - #4 by andrea1
Incoming calls don't appear on my screen - Fairphone 2 - iFixit
Can anybody help suggest what the issue is and how to fix it? Thanks in advance for any pointers.

Some kind of problem with the device itself. And before this situation, did the phone fall down? Did you spill any water on it?

You mean July 2022 ??

How often does it happen?

Try use the phone in Safe Mode for a while.

You don’t say, so are you using the default FOS updated 0021 Feb 5th Security patch?