I recently bought a used iPhone 4 online to use with AT&T in the United States. The model I bought appears to be from Canada (MC605CLL/A) and is locked to Bell (Canada Carrier.) I thought it would be factory unlocked but I guess not. Therefore, my AT&T SIM is not valid for this phone. My question is, would the Apple store exchange it for a AT&T locked model? The phone is under warranty for another year and appears to be in almost new condition. If not, I will simply just try to return it back to the seller.