My phone doesn't always ring for incoming calls or show missed calls

I'm having the exact same issue. I'm with Verizon and they (along with Apple) have no idea what could be causing this. All I can tell you is to contact your provider OR Apple and see about getting a replacement. That's what they're doing for me and I'm hoping it'll solve the problem but I don't think it will.

Good luck!

I am having this issue too. I have replaced the SIM card twice and Apple replaced my device. Two days later I was missing calls.... no notification at all only knew they called when they left a voicemail. I have verizon and have called at least once a month since April to resolve the issue and still having problems. If you find a solution to this problem please pass it along. Thanks.

These issues can be caused by applications like TRUECALLER the app isn't perfect and can set about blocking loads of numbers it shouldn't. Happened with me. Delete it and then go into the phone section of settings to see what's been blocked and unblock them if need be