Im getting this when i try to call. "We are sorry but this device is blocked from network use
Hi guys so I went to AT&T an asked why I am getting this error and I also asked if this phone is stolen. They said they don't know why that is happening and they also said the phone is by stolen. So the problem is thy whenever I put in a AT&T SIM card it will wok for 20 minutes then when you try to call it will say " we are sorry but this device is blocked from network use...." Then I would have to get a new sim. I would understand if this happens to a stolen iPhone but I asked AT&T and they said its not stolen or lost. So I am so confused. I tried restoring the phone a few times. By the way it's an iPhone 4S.
iPhone 4S, iOS 6.1.1