In a grand, provocative fashion, T-Mobile officially announced and launched its new 4G LTE network earlier today. As part of its new network, the carrier also unveiled new contract-free rate plans and that it will be offering the iPhone starting next month. Additionally, T-Mobile is essentially opening the door to customers of other carriers to bring their phones over — including their iPhones — and sign up for T-Mobile's more aggressively priced plans. But thanks to a variety of factors, you may not want to do that.
The biggest issue with bringing an AT&T iPhone 5 (we're going to use the iPhone for most of this article, but it can also apply to a number of AT&T smartphones) to T-Mobile is compatibility with T-Mobile's network. T-Mobile has been working feverishly to "upgrade" its network across the country to support the frequencies used by AT&T's iPhone, but that is limited to the slower, HSPA+ service, and does not apply to T-Mobile's faster 42Mbps HSPA+ network.
The current iPhone 5 won't play nice with T-Mobile HSPA+ 42
For example, if you were to own an iPhone 5 on AT&T, cancel your service, and by some stroke of luck, get AT&T to unlock it for you, it would only be compatible with T-Mobile's 21Mbps HSPA+ network in 49 cities and the handful of cities that now have T-Mobile's LTE coverage (outside of that, it will be limited to 2G EDGE service on the rest of T-Mobile's footprint).
Of course, if you happen to own a Sprint or Verizon iPhone 5, you are essentially in the same boat — neither of those models will work with T-Mobile's LTE service, but they are compatible with its limited HSPA+ service in those 49 cities.
But in the near future, that won’t be the case for new iPhone customers. Starting April 12th, the day that T-Mobile will start actively selling the iPhone, Apple will be selling versions of the iPhone 5 that are fully compatible with T-Mobile's network, even those that are purchased from AT&T. That means that whether you buy an iPhone from AT&T or from T-Mobile (but not Sprint or Verizon), you will be able to use it on either carrier's HSPA+ and LTE services, provided it is unlocked, of course. Unfortunately for those of us that currently own AT&T versions of the iPhone 5, Apple says that it will not be upgrading current units to be fully compatible with T-Mobile's network — the upgrade will only apply to new iPhones going forward.
Unlocked iPhones will be a little more portable
Apple says that it will be selling unlocked versions of the T-Mobile compatible iPhone 5 alongside the standard carrier locked models that T-Mobile will sell in stores. The current price for an unlocked iPhone 5 is $649 for a 16GB model. If you plan to use your iPhone 5 on T-Mobile exclusively, it's a bit cheaper to pay the $99.99 up front fee and the 24 $20 monthly installments (a grand total of $579.99) than to buy it unlocked. But for those of us that prefer or require an unlocked iPhone, we'll soon have more choices as to where we can use it.
Update: This article has been updated to better clarify Apple and T-Mobile's stance on LTE support for the current iPhone 5.
Update: Apple has reached out with a further clarification on the network compatibility of the existing iPhone 5: Customers that own the current iPhone 5 sold unlocked and for AT&T can use it on T-Mobile's HSPA+ 21Mbps and LTE networks, though it will not work on the carrier's HSPA+ 42Mbps network. Once T-Mobile starts selling the iPhone on April 12th, the iPhone 5 will be sold with support for both HSPA+ 42Mbps and LTE on T-Mobile's network. This article has been adjusted to account for the update.