I had to exchange an iPhone 4S about 2 months ago because no one could hear me talk on the phone without putting it on speaker phone, and when the apple rep came out, I saw him have to chose between the carriers that the iPhone 4S was on including at&t. So the question I have is, can I unlock this device and make it compatible with the at&t network and how do I do that? I have never unlocked a device before.