If you want to use an AT&T iPhone with T-Mobile or another wireless carrier in the US your only choice is to find a way to perform a “carrier unlock” on the phone, since the only US wireless providers that offer the iPhone are AT&T and Verizon. Up until recently, the only way to do that was to jailbreak your phone, wait for hackers to develop tools that would let you unlock the phone, and then the next time Apple rolls out a minor (or major) operating system update, avoid applying it until those hackers have figured out how to jailbreak and unlock the phone again.
Now there’s a simpler option, albeit one that’s a bit more expensive. A company called CutYourSim says it can issue an official unlock code for AT&T iPhone models for $169.99. Basically you send in your device IMEI and they send you a code. The process takes about 2-5 days.
The upshot is that you can unlock your phone without jailbreaking. That means you don’t void your warranty, and you can download Apple software updates to your heart’s content without any problems.
That’s because what CutYourSim is doing is adding your device’s unique code to Apple’s database as an unlocked device. It’s not entirely clear how this is done, and while some folks are wondering if the company illegally hacked into the database, others figure CutYourSim or one of the company’s partners simply has legitimate access to the database, as would any company that sells the iPhone.
On the other hand, as BGR points out it’s just as easy to remove a device from that database as to add it — so there’s no guarantee that CutYourSim’s unlock process is really forever.