Hello All! I was having problems with my phone and recently took it to the Apple store for diagnosis. They took my AT&T locked phone and issued me what appears to be a factory unlocked 64GB iPhone 4s - Model No. MD260LL/A. I've searched the web and everything coming up shows this model as a Factory Unlocked iPhone. Is this the case? When I put in my inactive T-mobile SIM card, I was told that activation is required and to insert a valid SIM. Is it possible the phone is still locked?