Editor's Note: This story was updated on Sept. 16 with new information.

If you're buying an iPhone 6s or 6s Plus and want to have the option of changing carriers to AT&T, you need to keep an eye on the model number. Apple is selling two different 6s models here in the U.S., one of which supports AT&T's new, faster Band 30 LTE network.

The AT&T model, which will also be sold by Boost, Cricket, U.S. Cellular, and Tracfone brands such as Net10 and Straight Talk, is A1633/1634. It supports all the major U.S. carriers including Verizon (and, in fact, all global carriers) except for one major catch: you may not be able to get it unlocked. None of the larger U.S. carriers listed as carrying the A1633 model will unlock the phone on purchase. AT&T, for instance, requires an account to be active for 60 days before the phone can be unlocked.

Everyone else is getting model A1687/88. (I've confirmed this with the three other carriers.) This is identical to A1633/34 except for Band 30 (which I've confirmed with Apple.) Both models work ideally on Sprint, T-Mobile and Verizon Wireless. Model A1687/88 will work fine on AT&T—it'll get LTE and everything—but as time passes and Band 30 is more heavily built out, it will show somewhat slower LTE speeds than the A1633/34 model.

Apple didn't comment on why there are two models, as apparently, it could have sold A1633/34 to everyone and had them be perfectly happy.

For the record, Verizon phones are unlocked if purchased at full price. T-Mobile iPhones are unlocked if purchased at full price directly from Apple (as opposed to from T-Mobile.) AT&T and Sprint phones are not unlocked by default. All the carriers will unlock your phone once you've signed up for service and used it for a certain amount of time, which varies by carrier and plan, but you do have to sign up for service. You can't get an iPhone without service from Apple right now.

There are two more sets of models that may be in play. Model A1699/1700 will only be sold in China. It appears to be identical to A1687/88. FCC documents also show that there is an A1690/91 model, which Apple has not yet discussed.

Band 30, also known as WCS or 2300MHz, accelerates LTE performance in some urban areas. According to Fierce Wireless, AT&T has already launched it in "a handful of markets" and plans to deploy more later this year.

If you want to pre-order an iPhone 6s, you have to choose a carrier unit. Choose the one for the carrier you intend to start with. If you switch to AT&T with the A1687/88 down the road, it'll still work.

If you really want the freedom to switch between carriers, wait a few weeks for the formally unlocked phones to appear. Right now, Apple tells me it is not selling formally unlocked iPhones by pre-order. "Apple may add it, but as of right now it is not available," a retail rep said.

Note: The history of this story is that Apple posted some erroneous spec sheets last week, confusing everybody. We wrote a story based on Apple's original spec sheet, and then updated the story periodically as Apple revised its website. I'm pretty sure we've reached the final state now.

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