i have a million dollar suggestion for u and others who need help in this regard, few of my friends did this and it worked, first they called atnt and asked to block internet from their lines (u can give any reason, son keeps on playing with it, wife surfs porn bla bla bla), so once internet is blocked in ur line, u can use the same sim card in an unlocked iphone, atnt can't find out that u r using an iphone,

this method has worked for other smartphones as well