Buy any non-Chinese model, and it will work over LTE (and regular networks) on any American carrier. There’s just one small difference with AT&T involving additional frequencies it has a long-term plan to deploy fully.
The confusion arises in part because the explanation and split among supported networks is somewhat different with the iPhone 6 and 6 Plus. With the 2014 phones, the model oriented toward AT&T, T-Mobile, and Verizon lacked some bands primarily used elsewhere in the world. That’s been resolved in the 6s/6s Plus, and Apple could be clearer about communicating it.
For several releases, Apple has made combined GSM/CDMA phones: the GSM standard is the dominant one used worldwide for 2G and 3G voice and data, and by AT&T and T-Mobile in the U.S. CDMA’s predominant deployment comes from Sprint and Verizon in America for 2G voice and 3G data. All four companies (and nearly all carriers worldwide) converged for next-generation networking on 4G LTE.
However, in almost no case have any two carriers leased the same frequency bands, even in the same country. LTE can be configured a number of ways, which is one reason it caught on so fast: it can use very narrow and very broad swaths of spectrum, where 2G and 3G had much more rigid requirements. As a result, many more bands are in use, and cellular radio chipmakers and handset makers keep squeezing more support in with every new release of silicon or phone.
The confusion about the iPhone 6s/6s Plus models arises from how Apple presents LTE support on its technical marketing page. It uses the phrase “Supported LTE Networks” in its description of each of the various network-specific model phones it sells. But despite what that implies, its two primary global 6s/6s Plus models currently work on all networks with equal support.
Apple has model numbers for its phones, even though you rarely need to use them:
While the A1688 and A1687 are also marked as GSM or CDMA, the models are actually identical: they are activated for one or the other, but can be used on either kind of network. CDMA doesn’t rely on a SIM (Subscriber Identity Module) like GSM phones, and carriers have more control over activation. However, all new “CDMA” iPhones include a GSM SIM slot, and have since 2011.
The only difference between the AT&T and “everyone else” models is support for LTE Band 30, which occupies frequencies starting at 2300MHz (2.3GHz). (The cell industry numbers LTE bands in addition to the frequency range for clarity and succinctness, because of the many possible configurations of LTE.) This is a fresh swath of spectrum for AT&T that it plans to roll out over time. In early September, AT&T spoke to FierceWireless and other publications about its plans to add capacity in urban areas using this bandwidth range, which it acquired in 2012 and continues to purchase more of. (Macworld has a query out to AT&T for additional detail.)
As of early September, only two markets had Band 30 switched on, and AT&T hasn’t clarified how quickly they plan to add service in more regions. Rather than offering better LTE rates, Band 30 will reduce congestion, according to the company’s earlier remarks. AT&T previously deployed LTE over 700MHz, which was abundant after being retired from use in analog UHF television broadcasts. This lower-frequency band penetrates buildings and can span greater distances.
However, in cellular networks, having smaller coverage cells allows greater density. The higher-frequency 2.3GHz band works better among buildings outdoors, as well as in corporate, convention center, and airport indoor deployments.
Something on the order of 100 million phones in the U.S. are equipped with LTE and lack Band 30 support, so while it’s important for future models, AT&T is expanding into this new frequency. Having a phone capable of it on AT&T’s network may allow you to have consistently higher throughput. Only a handful of new phones support it, including the Samsung Galaxy S6 and S6 Edge.
If you purchase a SIM-free iPhone from Apple, you get the A1633 or A1634, and thus are future-proofed against using it on AT&T or networks in other countries that could obtain the same frequencies.
As to why Apple would manufacture two sets of ostensibly almost identical phones that they sell at the same price There may be a very slight cost per unit difference in the radio baseband chips required to add Base 30 support, but unless it’s significant, it would seem like more trouble to Apple to maintain two similar lines. (Apple declined to comment on that.)
If you’re buying an iPhone to activate on a network other than AT&T, you’ll get the model that lacks Band 30. Bringing it later to AT&T won’t cramp your style, but you won’t be able to take full advantage. On the other hand, as people migrate to newer phones that opt for Band 30 when it’s less crowded, fewer phones will be stuck on older bands, which should then see improved performance.