I just purchased a Galaxy S III at my AT&T corporate store and realized that I had several little scratches on and above the ear piece.  I immediately took it back the next day and spoke to the store manager. She said I am allowed one exchange and swapped it out for a new new one. 

You should go to an AT&T corporate store and demand they exchange it for a new one. You are allowed one in store exchange PERIOD.  Now if this was the second defective phone than yes, you'd have to go through the warranty. 

Keep trying and call the AT&T resolution department if you have to. Just tell them you are going to cancel your service and you'll be transferred over there. 

Jam the AT&T forum community leader will definitely help you also. PM her. She's helped me in the past and is very good at what she does. 

Best of luck!!!