I purchased an iPhone 4S last year on release date as an upgrade on my AT&T line. A month ago I was having issues with the phone so I got a warranty replacement at the apple store. I work at T-Mobile and was playing around with my phone today and I put a T-Mobile microsim in the phone and it worked. Has anyone else experienced this? A search on macrumors says that apple gives like for like but mine is unlocked.