For those of you who have a Lumia 900 on AT&T, you might be interested in this. According to WPCentral, you can call AT&T with your International Mobile Equipment Identity (IMEI) and get your unlock code. This unlock code will allow you to use your phone on any network. AT&T has verified they are unlocking phones, but with a few stipulations. In order to get the unlock code, you’ll need to be out of your contract or have paid the ETF fee, otherwise they won’t release the code.
People have had success in calling AT&T to get their unlock code, but of course, your mileage may vary. If you’re interested in trying this method, you can dial *#06# to obtain your IMEI before you call AT&T and then attempt to coax them into giving you the unlock code for your phone.