Do you have a ringtone set for incoming calls? Can you adjust the volume while a call is coming in and hear any tone at all?
I've seen people go in and play with their ringtones and end up setting the tone to silent by mistake and then like you end up with a device that doesn't ring or sound when calls come in. I've also seen, people use apps that provide ringtones or other customizations, like backgrounds and icons, and then the app is pulled and everything provided goes with it.
Have you tried anything? So what happens if you press the Vol + button while the phone is ringing? If you are sure you have a ringtone then this is an important test. If the volume increases to an audible tone. What happens if another call comes in? I'm not trying to be funny but are you certain that your notification volume is what you are adjusting? Most devices have 3, 4 or even 5 volume settings. If you're playing a game, listening to music or some other media thing adjusting the volume will often adjust the media volume. If you are on the call there is an in-call volume. If you have alarms there is an alarm volume. Then there can be a notification volume. Some devices will group these and some will break them down. Touching either volume button and then using the pull down arrow to the side will often show you all of your volume options.
If you have a tone, the volume is set when the phone rings, but there is still no sound the only thing I can think of is a launcher. Are you using a launcher or anything that changes your dialer from stock? Dialers and launchers are notoriously buggy. Yeah they look great but they often affect the functionality of devices, which is kind of what they were made for but the effects aren't always desirable.