I had this problem when I first joined AT&T over 2 years ago. It had nothing to do with the phone I had at that particular time. It was some kind of error with my account.
I spent a lot of time talking to customer service on the phone and in the store. They finally switched my number and sim card and re-activated my account. It has been fine every since.
I would go into a store with a friend that has a different carrier. Then you can have your friend text you and have them sort it out right there.Thats what I ended up doing.
Good luck.