some incoming iPhone calls do not notify me even though I do not have do not disturb enabled

Frequently I am waiting for a call and my phone never rings or notifies me that the call is incoming. I only discover I missed the call when I look at Recent calls and see a number in red. Frustrating. No, "Do Not Disturb" is not enabled. This doesn't happen on all incoming calls, just a few, mostly numbers that are not in my directory. Would it help if I restored my iPhone back to default?