When AT&T was hogging the iPhone, “unlock your iPhone” ads were everywhere. Verizon, Sprint, and T Mobile customers wanted in on the action, too. Why should AT&T customers have all the fun?

But now that pretty much every carrier has the iPhone, wouldn’t you know it? AT&T has finally agreed to let out-of-contract users put other carrier’s SIM cards in the iPhones they purchased through AT&T. They’re letting users move the phones they bought to another carrier. Gee, how generous.

It should be noted that if you bought your iPhone through AT&T, you won’t be able to use it on Sprint or Verizon, since they use CDMA networks. It will, however, work with T Mobile’s network. Users report speeds slower than 3G on unlocked AT&T iPhones using T Mobile’s network, so beware.

The biggest benefit to AT&T iPhone users will be for overseas travel.

An unlocked iPhone is ideal for users who are traveling overseas, because instead of paying for expensive service from AT&T, they can pop in a cheaper SIM card from a local GSM wireless carrier,” Jared Newman of PC World says.

Sprint and Verizon already allow users to unlock their phones for overseas travel, Newman points out, as long as the user’s account is in good standing at the time. AT&T will allow iPhone users not currently under contract to unlock their phones, but those still in contract will have to pay an early termination fee.

Until recently, the only way to unlock an AT&T iPhone was to “jailbreak” it, a term that applies to altering the functionality of a device by gaining root access to its operating system. iOS jailbreaking has most recently become newsworthy as iPhone owners seek to put Siri on older iPhone models.

iOS jailbreaking is also used to switch carriers, with its popularity swelling in the days before every carrier sold the iPhone. Some thought the practice might be illegal, but the Federal government ruled it was okay. Apple has never been too happy with the practice, reporting in 2009 that more than 400,000 iPhone users had attempted iOS jailbreaking.

Apple has continued to warn that jailbreaking can damage the phone and void the warranty.

In fact, the company has reportedly said it receives numerous complaints about the damage jailbreaking can do. It’s definitely something that should be done with care, as permanent damage can be done to your phone if you put your jailbreaking in the wrong hands.

But if unlocking is all you’re interested in, you can leave it up to the experts at AT&T.

If you’d like to have your iPhone unlocked, follow these steps.

• Either call your cell phone carrier or visit a retail location. They will use the IMEI number on your phone to determine eligibility. Your IMEI number can be found under Settings-General-About. Scroll down and you’ll find it. Your carrier will check your eligibility and, if you qualify, they’ll let you proceed to the next step.
• You will be notified via e-mail once your phone has been unlocked. You will then need to connect your phone to iTunes, at which point you’ll step through the provided instructions to backup and restore your phone to default factory settings. This will prepare your phone for the next step.
• You will need to contact your new cell phone provider to get a SIM card for your device. Once you have the SIM card, you will need to look for a pinhole opening on the side of your phone opposite the volume controls. Inserting a paper clip into that opening causes the SIM card to be revealed, allowing you to remove it and switch it for another.

Be aware that your new SIM card will give you a new phone number—not a big deal if you’re unlocking your phone for use overseas. The good news is, International pay-as-you-go SIM cards are available online from AT&T Go Phone. Once you’ve bought the card and slid it into your phone, you should be able to begin purchasing minutes.

Unlocking can also be helpful if you purchase a full-price iPhone on Craigslist or eBay that’s locked to a particular carrier. This frees you up to purchase a wider range of shopping options in the used phone market. Check with your carrier to make sure your contract will let you unlock the phone before you purchase it.