If you want to buy an iPhone 5 and enjoy 4G speeds all around the world, you may have to buy it from Verizon or Sprint. That’s because the LTE chip in AT&T’s version of the iPhone 5 is far more limited than its competitors.

Any iPhone 5 will be able to roam internationally by falling back to 3G networks, but Verizon and Sprint iPhones will enjoy 4G LTE speeds throughout much of the globe because they support five LTE bands. According to Apple, the AT&T iPhone 5 will support only two LTE bands:

The difference is crucial. If you cross-reference the LTE bands shown above with a list of LTE band usage by country, you find that the AT&T iPhone 5 will be able to access LTE networks only in the US, Canada, and Puerto Rico. The Verizon and Sprint iPhone 5 will get LTE speeds in the US as well as in at least some—but perhaps not all—of these countries: UK, Philippines, Sweden, Angola, Australia, Belgium, Czech Republic, Denmark, Estonia, Finland, Germany, Hong Kong, Hungary, Japan, Latvia, Lithuania, Namibia, Poland, Singapore, South Korea, Uzbekistan, and the United Arab Emirates.

UK speeds may particularly be a problem, because the Verizon/Sprint iPhone supports the LTE band used by Everything Everywhere, but not the bands that will be used by the UK's three other major carriers.

Nor will the Verizon and Sprint versions be compatible with LTE networks in Canada, it turns out. If you shuttle between the US and Canada but never go further afield, AT&T may still be the best choice. However, as a plus for T-Mobile, the AT&T unit will support T-Mobile's imminent LTE rollout, giving the prospect of better-than-EDGE speeds on that network for the first time (if you can buy an unlocked phone and use it with T-Mobile). The Verizon/Sprint iPhone model will not support the LTE band to be used by T-Mobile.

The same version of the iPhone 5 sold by AT&T will be sold by carriers in Canada. A third version to be sold outside North America supports three LTE bands.

All the iPhones will have 3G capability, of course. Apple’s iPhone 5 tech specs classify the AT&T version as a GSM model, and the one sold by Sprint and Verizon as a CDMA model. All of them support UMTS and HSPA+ standards.

Why such an LTE discrepancy exists between the models sold by different carriers hasn’t been explained. We’ve contacted Apple, Sprint, Verizon, and AT&T to ask if there are technical reasons. We’ve also asked Sprint and Verizon if they have roaming agreements in place to take advantage of the CDMA iPhone’s international LTE roaming capabilities. We'll provide an update if we get more information. Clearly, the iPhone's move to LTE will not be simple when it comes to using the next-generation network while overseas.

UPDATE: Verizon got back to us, and said "Verizon Wireless plans to enable global LTE roaming on the iPhone 5 in the future. As there are many LTE frequencies currently being deployed around the world, Verizon will be surveying which markets line up best with the frequencies available in our version of the iPhone 5."