T-Mobile is the only major carrier in the US to not offer Apple’s ubiquitous iPhone, but that doesn’t mean that it ignores customers that wish to use the popular device. In fact, the carrier already offers micro-SIM cards and technical support to customers with iPhones and in a blog post today, T-Mobile details how it is ramping up its efforts to cater to more iPhone customers with better support, training, and services.

T-Mobile has long said that more than one million of its customers are already using iPhones, but it seems that the carrier would like to increase that figure — even though it still doesn’t sell the phone directly. Starting on September 12th (coincidentally, the same day as Apple’s big launch event), T-Mobile retail staff will be trained on setting up iPhones for customers and each store will have iPhone 4S demo units on hand. The carrier is also developing iOS versions of its popular mobile apps, such as T-Mobile myAccount, T-Mobile Visual Voicemail, and T-Mobile TV, which will soon be available to download in the iTunes App Store.

The biggest barrier for an iPhone user moving to T-Mobile is most certainly the upfront cost of an unlocked device: the current iPhone 4S starts at $649 for an unlocked model ($450 more than a subsidized model), and the next version will likely be in the similar price range. Additionally, since the iPhone is not officially offered by T-Mobile, customers will have to buy their phones directly from Apple and then bring them to a T-Mobile location to sign up for a plan — adding another step to the already dreaded process of purchasing a smartphone and service. Add to all of this the fact that the iPhone only gets 2G EDGE data service on the vast majority of T-Mobile’s network (and the next iPhone will very likely support LTE service on AT&T, Verizon, and possibly Sprint), and you can see why most iPhone customers just buy their phones from one of the other three major carriers.

But T-Mobile plans to be aggressive in pointing out certain advantages it has over the other big three. The carrier recently launched a truly unlimited plan, giving it an edge over AT&T and Verizon that only have tiered plans available at this point. T-Mobile will also advertise that its plans can save unlocked iPhone users $50 per month over comparable options on AT&T. Incentive offers to iPhone users in select markets are in the works as well, though T-Mobile doesn’t detail what they would be specifically. Traditionally, T-mobile has used AWS spectrum for its 3G data service, which is not supported by the iPhone. As a result, iPhone users on T-Mobile have had to put up with the carrier’s slow 2G EDGE data service, which is much slower than the 3G and 4G networks run by the other carriers.

To address this problem, T-Mobile is lighting up 4G HSPA+ service on the 1900MHz frequency (which is supported by the iPhone) in more locations as part of its eventual transition to LTE. If you’ll recall, the carrier announced a public test of its HSPA+ service on 1900MHz in San Francisco during Apple’s Worldwide Developer Conference in June, marking the first time that T-Mobile made its 1900MHz HSPA+ tests public knowledge. The carrier notes that 1900MHz HSPA+ is now also available in Seattle, Las Vegas, and the New York metro area, and it will be deployed in more markets before the end of the year.

Unfortunately for existing T-Mobile customers, the carrier’s plans to aggressively pursue those with unlocked iPhones all but guarantees that it will be left out of the official iPhone party later this week. T-Mobile has been at a great disadvantage to the other carriers in the US ever since the iPhone was released, and the subscriber losses it has suffered in recent quarters have reflected that. Chances are customers won’t be flocking to T-Mobile with unlocked iPhones in their hands this fall either, but if they do, the carrier plans to be ready for them.