AT&T (News - Alert) has announced that it is unlocking iPhones for customers who have fulfilled their contract requirements with AT&T and have an account in good standing.

“Beginning Sunday, April 8, we will offer qualifying customers the ability to unlock their AT&T iPhones,” said the AT&T statement. “The only requirements are that a customer's account must be in good standing, their device cannot be associated with a current and active term commitment on an AT&T customer account, and they need to have fulfilled their contract term, upgraded under one of our upgrade policies or paid an early termination fee.”

Customers who meet the requirements will simply have to contact AT&T to request the carrier unlock. Once unlocked, the phone will be able to use any micro-SIM card, provided that the carrier frequencies are supported. This means easy linking to internationally-based phone providers.

Currently, AT&T sells unlocked iPhones at a price of $649 for a 16GB phone. The company does subsidize the cost of the phone to bring it down to around $200, but, when they agree to pay the reduced price, customers are locked into a contract with AT&T. Customers can purchase unlocked iPhones directly from Apple without striking a deal with a carrier, but they will pay the full price for the phone.

Unlocked phones allow for the insertion of any micro SIM card from any GSM carrier into the phone. In the United States, that means AT&T and T-Mobile. However, T-Mobile (News - Alert) operates on a different frequency, meaning that users only receive input at 2G, or “EDGE” speeds. Overseas, customers can use micro SIM cards from carriers like China Mobile, Orange (News - Alert) and Vodafone. When users sign up for a plan, they can request a corresponding micro SIM card from the provider.

If carriers do not offer micro-SIM cards, which are the only size accepted by iPhone (News - Alert) 4 and 4S, users can purchase full-size SIM cards and trim them to fit into the micro slot. If users want to transfer their micro-SIM card from their iPhone 4 or 4S to an older iPhone, then they can purchase an adapter that allows them to plug a micro-SIM into a full-size slot.

AT&T has had a long-standing policy of refusing to unlock subscribers’ iPhones. Verizon and Sprint (News - Alert) do allow unlocking for international use, and they allow it for users who are currently in contract.