It sounds like your proximity sensor might be bad which would be covered by the factory warranty. A quick fix would be an app like screen suite which locks the screen while you are on a call so you can't hang up on anyone but I would probably just take it into Verizon and have it replaced if that sensor is actually the issue. I've heard that long hair can cause this problem also
I agree with Warrier. It's probably the proximity sensor. One thing you can do on the phone however is to check and make sure that under Menu/Settings/Call Settings/Keep Screen Awake is not checked.