IPhone - Can't hear incoming calls. Does not ring or Vibrate.

I had a similar problem....it was usually after I unplugged the headphones....the phone thought they were still attached, therefore, nothing came over the speaker......try plugging in the headphones, then unplugging...that worked for me.

If the iphone still does this after a hard reset and a software restore, it's usually caused by a defective audio jack module (swap iphone backplate) or the electronic module that sits behind and drives the earspeaker. In both cases this should be covered under warranty. If you're not covered, you can try an iphone repair guide websites, there are also places that sell iphone parts.