If you purchased your iPhone for less than $300 when new, or if you didn’t consciously purchase an “unlocked” phone, you can pretty much guarantee that it was subsidized by the carrier, and is therefore “locked” and can only be used by their network and their SIM cards. Not only does this limit your ability to move between domestic carriers, it also prohibits you from using SIM cards for carriers in other countries while traveling.
The easiest way to tell whether or not your iPhone is locked is by simply inserting a SIM from a different carrier. If it’s locked, you will receive an error message. Short of this, there is also another way to tell whether your iPhone is still locked. dev.mk offers a service that allows you to type your IMEI number in to their web site, then they’ll tell you whether or not it’s locked or unlocked. You can take their service one step further however. dev.mk ultimately uses Apple’s Global Service Exchange (more commonly known as GSX) web site to obtain information from
Apple’s servers to determine whether or not your iPhone is locked or unlocked. GSX is not a public web site, but is rather a system used by every Apple Authorized Service Dealer in the world, including the Apple Store and Apple Authorized Resellers, to manage repairs of Apple products. You can therefore contact your local Apple Store or Apple Dealer to obtain this information. I’m sure your local Apple Reseller will even do this by email for you – they’ll just need either your iPhone’s serial number or IMEI number. I have attached a couple sample screen shots to show you exactly what data GSX will provide.
If you have a locked iPhone, there are services and add-in circuit boards to allow you to circumvent the carrier locking, but I have never been a fan of them as they are only successful for a short period of time before Apple creates a new software update to disable them. I’m also very suspicious of the third-party services that offer unlocking of the IMEI. From what I’ve read, some of these services involve having a mole working at the carriers on their behalf, and therefore the unlock could be reverted by the carrier if the leak is discovered.
The better way to go is instead to pay your carrier directly to unlock your phone. Depending upon how deep you are in your contract, you may or may not be allowed by your carrier to do this. This engadget article outlines the costs for each carrier (ranging from $35 to $75) and links to instructions. For US readers, AT&T has a handy unlock request form here. To have the freedom to switch from the carrier you bought your phone from, to another carrier, your iPhone must be “unlocked”. Your carrier collects your fee, presses a few buttons in their system, and your phone is permanently “unlocked” and able to be used with any carrier’s SIM cards.
If you intend on traveling internationally, shell out the extra money (usually about $500 on the iPhone) for an unlocked iPhone, and the freedom of being able to roam with international SIM cards will easily pay for itself on your first trip.