I spent the first 2 weeks of March working on this exact same issue with an Apple Tech.
It infuriates me that I miss calls just because I am looking at a website or checking email or even looking at Facebook.
I contacted Verizon wireless a couple of times but they weren’t helping me at all, so I contacted Apple support.
The second Apple support tech that I spoke to decided to (take ownership) of my problem, and I spent almost every single day for the first 2 weeks of March with her going over every setting on my phone, and remotely getting onto my phone To help me navigate the system to try to find the problem, having me download files that she would send to me that would remote run remote tests and record actions on my phone when She would have me do certain things to try to re-create the problem while I was working with her. Then I would send those recordings back to her and she would submit them to Apple‘s engineers, and they couldn’t find anything.
So then she made an appointment for me to go to the nearest Apple Store an hours drive away from me, and they ran some tests on my phone and couldn’t find anything wrong either.
I told every single one of those people that I talk to that I’ve only had this problem since I installed their latest update.
I even told them all about forums like this where everybody is complaining about the same issues. But they said they haven’t been receiving very many calls So they don’t think it’s a widespread issue.
It would really help if every single person on this feed would call Apple and complain.