Welcome to Android Central! Which phone? Are you sure the Ringtones volume is turned up? Go to Settings>Sound, and look at the different volume sliders. It's possible for your Media volume to be turned up, but your Ringtones volume to be turned all the way down.
The same thing just started happening to me. My ringtone volume is turned as loud as possible, but the phone doesn't even seem to ring, I just get a notification at I missed the call. I have a Galaxy 8, android