T-Mobile USA, the only "Big 4" phone company that doesn't sell the iPhone, now wants to snag used ones from AT&T.

Starting Wednesday, when Apple is expected to reveal a new iPhone model, T-Mobile will start advertising that AT&T iPhone owners who are out of contract can switch to T-Mobile.

"We expect that consumers will start trading in older devices," said Harry Thomas, T-Mobile's director of marketing. "For every person waiting in line for the next model, a lot of them have to find a secondary market for that older device."

In New York and Atlanta, T-Mobile will be giving away $100 gift cards to customers who sign their iPhones up for two-year contracts.

in a few spots in such cities as New York, Seattle, Las Vegas and Washington.

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AT&T iPhones have to be unlocked using codes that AT&T will supply after the customer's contract is up. Sprint and Verizon iPhones of the "4" model won't work on T-Mobile's network at all. The later iPhone 4S will work if it's been hacked, but that's not something T-Mobile wants to get into.

T-Mobile's pitch will center on its unlimited data plan. AT&T stopped signing up new customers to unlimited plans two years ago, and now slows down service for the rest of the billing cycle once those still on old unlimited plans reach a certain level of data usage.