iPhone has registered 3 missed calls.
my iPhone 8 has registered 3 missed calls, but I haven't had a ring tone to answer them.
I don't think I've set it correctly to hear the ring tones. Volume is set to near maximum.
Go to Settings>Phone>Silence Unknown Callers and make sure that it's set to off.
Make sure that you don't have Do Not Disturb enabled. Settings>Do Not Disturb.
Make sure that you have not engaged the mute switch. It's the little slider above the volume buttons.