Setting the record straight: Own an AT&T iPhone 5? It will work on T-Mobile’s LTE network

There have been a lot of conflicting and confusing – and several plain wrong – reports on whether the current version of the iPhone 5 will work on T-Mobile’s new LTE network. I’m sorry to say I even helped spread some of that misinformation by talking about those reports on GigaOM’s mobile call-in podcast on Wednesday. But I’ve since had a chance to talk Apple(s aapl), and got the details about what exactly the iPhone 5 can do and what it can’t.

Bottom line: if you have a North American GSM version of the iPhone 5 — whether you bought it from AT&T or Apple or got it in from Canada – it can connect to T-Mobile’s new LTE network. It just has to be unlocked. So for AT&T(s t) customers looking to switch sides, that means you have to finish your contract, and ask your carrier to unlock the device.

iPhone 5 Lightning dock connectorThe device will also work on T-Mobile’s 3G HSPA+ network (which T-Mobile calls 4G), just not in every city today. T-Mobile is in the process of a big network overhaul that will align all of its networks with the radios in the iPhone and most other AT&T devices. It’s completed the upgrade in about 50 cities covering 142 million people, but other cities are getting converted quickly.

The source of the confusion is over frequencies, which is why we’ve been seeing all of these references to the Advanced Wireless Services (AWS) band. T-Mobile runs two technologies in the AWS band, it’s LTE network and a portion of its HSPA+ network. The iPhone 5 will support LTE in the AWS band, but it won’t support HSPA+ over AWS. The iPhone 5, and all previous versions of the iPhone, will work on its new upgraded HSPA+ systems in the PCS band.

Apple will release a new version of the iPhone 5 next month that will make all of the band differences completely moot. The updated version will support HSPA+ on both AWS and PCS band. It will even be able to access T-Mobile’s dual-carrier 42 Mbps HSPA+ network, which current and older versions of the iPhone cannot.

All of this is probably still extremely confusing so I’ve broken it down into a Q&A, which hopefully will answer any lingering questions.

How do I know if my iPhone will work on T-Mobile’s networks?

For the iPhone 5, check your model number. It must be the A1428, sold by AT&T, the Canadian operators or Apple. Older iPhone models will also work on T-Mobile’s 2G and 3G networks. All of these devices must be unlocked, though, or they’ll be blocked.

How can I be sure I’ll have access to T-Mobile’s LTE and HSPA+ networks?

For LTE, it’s simple. T-Mobile launched LTE in seven markets this week: Baltimore, Houston, Kansas City, Las Vegas, Phoenix, San Jose, and Washington, D.C. New York City is scheduled to come online this summer along with a bunch of other yet unnamed cities.

For HSPA+, it’s a bit more difficult to tell since T-Mobile doesn’t have any kind of map that tracks which markets have HSPA+ running on the PCS band. They make regular updates on their blog and to the media. PCMag has the most the recent list of T-Mo’s 49 iPhone-optimized cities.

As a general rule of thumb, though, if T-Mobile has LTE in your city, then HSPA+ will be in all the right places, too. And if you get a new version of the iPhone 5 next month, it will work on all of T-Mobile’s network.

When will I be able to bring my old iPhone over to T-Mobile? 

You can do it right now if you like. T-Mobile already has millions of iPhones on its network, running over its 2G and 3G services. In order to access LTE though, you’ll have to wait until Apple updates iOS, authorizing the iPhone 5 to use T-Mobile’s network. Apple hasn’t given a date for when this will happen, saying it will come as an over-the-air update.

Will I be able to access dual-carrier HSPA+?

On a current iPhone, the fastest 3G network you’ll have access to is its 21 Mbps single-carrier system, since all of T-Mo’s dual-carriers are in the AWS band. T-Mobile will eventually launch dual-carrier in the PCS band, but that will take some time. It has to close its acquisition of MetroPCS(s pcs) and convert a lot of old GSM networks to 3G first. If you’re set on accessing the dual-carrier network in the near future, then you’ll need to get one of new versions of the iPhone 5.