As the only U.S. carrier not officially offering Apple’s iPhone, T-Mobile recently began opening up its network to unlocked iPhones. Today, the company laid out its plans to court unlocked iPhone owners, which includes showing off iPhones in stores and promoting its inexpensive mobile plans.

T-Mobile says it will have iPhone 4S units in stores starting September 12, the same day as Apple’s iPhone 5 event. As for enticing offers, the company will push its $69.99 unlimited mobile plan, which is $50 cheaper than AT&T’s 5 gigabyte data plan with unlimited talk and text.

T-Mobile already has more than 1 million unlocked iPhones on its network, according to its latest earnings report. But after losing 557,000 subscribers last quarter, the company desperately needs to do something to stem its losses. Until T-Mobile officially gets the iPhone (which likely isn’t possible until it rolls out its LTE network next year), the best the company can do is promote itself as an inexpensive option for unlocked iPhones.

It’s a noble effort, but the big problem for T-Mobile is that iPhones will only receive 2G Edge data across most of its network. The company has begun rolling out its faster HSPA+ 4G/3G spectrum in New York City, Last Vegas, Seattle, and Washington, D.C., but iPhone users outside of those areas are sore out of luck. On average, T-Mobile claims its HSPA+ network gets 70 percent faster speeds than AT&T’s.