I bought mine last year when it came out and have had no issues. Could be related to the carrier, I would speak with them first.
If Apple determines the iphone to be defective, then they will replace if it is inside the warranty period. No one here can tell you what Apple will determine.
It sounds to me like your iphone is not defective. Newer iphones are no different than older ones. Not sure that that would make any difference.
You can make an appointment at the genius bar and see what they say if you like.