I don’t have an iPhone, but my wife does. The other day she handed it to me and asked if I could figure out why she couldn’t hear it ring. She says she checked all the settings; notifications, speaker volume, even the do not disturb feature. After looking at the phone a minute the answer was pretty simple. There is a tiny switch on the side of the phone near the top that silences the phone. If the switch has been activated (pointing down) the phone is silenced. Push the switch to the up position and bingo the ringer and all the various notifications that you have set will be audible. She’s happy again.
Switch up, normal (not silenced) mode.
Switch down, not normal (silenced) mode.