T-Mobile USA remains the only major U.S. carrier that doesn’t sell the iPhone, but the company is launching a new campaign designed to get more U.S. wireless customers to bring unlocked iPhones to its network.

The company will put iPhone 4S demo units in each of its retail stores, with staffers trained to help customers set up compatible iPhones on the network.  T-Mobile says it’s also working on iOS versions of applications including T-Mobile myAccount, T-Mobile Visual Voicemail and T-Mobile TV. And the company will start testing “incentive offers” in selected markets to court more iPhone users.

The new campaign starts Wednesday. T-Mobile will also be taking out ads to court iPhone users. The independent TmoNews blog first reported on the plan last week, but it was unconfirmed by the company until today.

T-Mobile says more than 1 million iPhones currently run on its network, citing cost savings of $50 compared with a similar plans on AT&T. The company offers unlimited data plans to new customers, unlike AT&T and Verizon, which have been shifting instead to shared data plans.

Customers on T-Mobile can use unlocked versions of iPhones designed for AT&T’s network; iPhones made for Verizon and Sprint won’t work on T-Mobile. The company says it has turned on its 4G HSPA+ wireless service iPhone-friendly 1900 MHz spectrum in cities including Seattle, Las Vegas, New York and Washington D.C., in advance of a broader rollout.

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