Receiving calls on iPhone

When someone calls me, sometimes my phone rings (or vibrates if it's on vibrate) and other times it doesn't, so I am missing calls. The only way I know that I got a call is that it vibrates once (I'm usually on vibrate) and when I look, it says "Missed call" or "Voicemail." I'm wondering whether the difference is that if someone is one of my contacts, the phone rings properly but if they aren't, it doesn't. I don't know whether that's the explanation and, if so, how to fix it.

I had previously asked about "announce callers" because I wondered whether that was somehow contributing to the problem.

Anyway, I see that "silence unknown callers" had been on; I don't know why. I never turned it on. I've corrected that and hopefully it will solve my problem.

Many thanks!!!