The iPhone SE comes in two hardware models in the U.S., and neither one will work on all four U.S. carriers. That means you need to be aware of which smaller iPhone you buy, based on your carrier of choice.

The problem, as it is with so many phones, is support for different LTE bands. You can see Apple's official band list on its website.

The AT&T, T-Mobile, and Verizon units, as well as the unlocked unit, do not work well on Sprint or on Canadian carriers. These units are model A1662. It has the LTE bands for AT&T (2/4/5/17/29), T-Mobile (2/4/12), and Verizon (4/13), but not all of Sprint's bands. Sprint's high-speed Band 41, also known as Spark, is missing. While you will get some Sprint LTE signal on bands 25 and 26, it won't be the best speeds.

Some international roaming bands are also missing from that unit. The A1662 model doesn't have Band 7, which is used for high-speed networks in Canada and Russia, among other places. A1662 will get LTE signal globally, but won't hit its maximum speeds in some of those countries.

Sprint users and Canadians will get model A1723, but that won't solve all of Americans' problems. While A1723 has a wider range of LTE bands, including seven for roaming and 41 for Sprint Spark, it lacks band 13, so it won't work on Verizon's LTE network. It also lacks band 29, so it won't get the best possible speeds on AT&T. It's fine for T-Mobile.

That puts Americans who want to carrier-hop—or border-hop—in a tough position, as no one unit will work ideally on every U.S. and global carrier. This is different from the Apple iPhone 6s, where all the currently sold models support all of the U.S. carriers equally.

When I asked Apple about this, it pointed out that this is a lower-cost iPhone. The modem speeds in the iPhone SE are also in between the 5s and the 6s ( at Amazon) —they're more like the iPhone 6, according to Apple.

That said, Sprint and T-Mobile users will see dramatic improvement in their network connections with the iPhone SE over the iPhone 5s. The SE supports T-Mobile's Band 12, which greatly improves LTE range and wasn't present on the 5s. It also supports Sprint Spark, which greatly improves speed and wasn't on the 5s. According to our tests, if the SE has the same characteristics as the iPhone 6, it should be about 50 percent faster than the iPhone 5s on Sprint.

Just make sure to get the model that's right for your carrier.

Sascha Segan is live from Apple with his hands on impressions of the iPhone SE and new iPad! Posted by PCMag on Monday, March 21, 2016

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