My two-year contract with AT&T recently came to and end and I decided to take them up on their recently announced ability to unlock an out-of-contract iPhone. If for no other reason than to make my phone easier to sell to someone who may want to use it on another carrier. Note that this is NOT a jailbreak and has nothing to do with jailbreaking. This process was performed on an un-jailbroken phone that was out of contract (meaning I've paid in full for the phone and now completely own it).
There's a lot of confusion about this process and how it works. There's also no way to verify that your phone is in fact unlocked. Thanks to T-Mobile, you now can for 99 cents. Here's how:
Step 1: Request the Unlock From AT&T
I did this via webchat and the AT&T rep was quick to understand what i wanted and performed the unlock right away.
Step 2: Backup and Restore Your Phone Via ITunes
Some online forums lead me to believe that the communication from AT&T to Apple to my iTunes to my iPhone may take a few days, so i waited three days to perform my backup and restore. I also read on the Apple support forums that you'll receive a message upon successful carrier unlock but I did not.
Step 3: Order a Pay-as-you-go SIM From T-Mobile
You could use another carrier, but i found T-Mobile was having an online special offering a $6 discount. The regular price was $6.99 (with free shipping). Total price $0.99.
Step 4: Trim Your SIM to Make It a MicroSIM
I've read that if you have a T-Mobile store nearby (I do not) that the salespeople there may have a tool to do this for you. There's a few guides online that describe this process, if you're a DIYer like me. If you screw it up, you're out only a buck. I used my Swiss Army knife's to score the SIM then the pliers to snap off the extra pieces. Then used a file to grind and smooth the edges.
Step 5: Remove AT&T SIM and Insert Your T-Mobile SIM
Once you're done trimming simply replace the AT&T MicroSIM in the SIM tray and insert your new SIM. After the phone is done syncing up you'll see "T-Mobile" in the carrier header section of your phone (instead of AT&T). If you receive an "Invalid SIM" message your phone isn't unlocked. I didn't even bother to active the new SIM.
Step 6: Enjoy Carrier Freedom!
That's all! I don't know how long T-Mobile will be offering the $0.99 deal, so even if your contract isn't quite done you should jump on this deal. My T-Mobile SIM has a nearly three-year shelf-life ("Activate by 4/20/2015").