I received my iphone 5 on Friday and initially was not able make or receive phone calls. I called at&t and they said it was due to the volume of activations. On Saturday morning, I was able to reset my phone and get the calling function to work. I soon discovered that while my phone displays 4g, i was unable to connect to cellular data: My phone displays the message: Could not activate cellular data -- You are not subscribed to a cellular network. After numerous phone calls to AT&T and apple and multiple resets and restores of my phone, a representative from AT&T advised me this morning that there is an known issue with the sim cards on AT&T phones purchased from apple that is causing cellular data not to work. She said replacing the sim card and having the phone reactivated will resolve this issue. I do not live in an area with an AT&T corporate store or Apple Store. I will be getting a new sim card on Tuesday. I will advise at this time, whether this works on not.