Will that unlocked phone work with your wireless carrier? Here’s how to tell

Google Pixel XL
Julian Chokkattu/Digital Trends

Carrier-subsidized phones are remnants of a bygone era. In the old days, you’d wait anxiously until your line became eligible for an upgrade, bolting fast as you could to your carrier’s store when the moment arrived. You’d compare the newest smartphones with the help of a sales clerk, and, after picking the right color combination and accessories, fork over a few benjamins to seal the deal.

The death knell for cell phone contracts sounded earlier this year, and the process has become a little less straightforward as a result. You can still stop by your local Verizon store and buy an on-contract iPhone, of course, but a growing number of devices never actually appear on retail shelves. Instead, they’re available unlocked online, either on a payment plan or for the full retail price. This has opened up a new world of choice when it comes to smartphones, but has also introduced some complications. Some unlocked phones work on Verizon, but aren’t compatible with AT&T, for instance. Others make calls and texts perfectly well on T-Mobile, but refuse to play nicely with Sprint.

Figuring out whether an off-contract phone is compatible with your network doesn’t have to be intimidating, hower. We’ve put together a guide that lays out all you need to know.

What’s an unlocked phone?

In the simplest of terms, an “unlocked” phone is a phone that’s compatible with one or more cellular networks.

Just because a smartphone’s sold “locked” doesn’t mean it’s locked forever, though. Many cellphones previously sold “exclusively” by a carrier have been unlocked to work with another carrier, a process that wasn’t always legal in the United States. In 2012, the Library of Congress ruled that the changes necessary to unlock a smartphone constituted a violation of carriers’ copyright under the Digital Millenium Copyright Act (DMCA). Luckily, that interpretation was superseded by the Unlocking Consumer Choice and Wireless Competition Act of 2013, which recognized unlocking as an exception to intellectual property protections.

Unlocked phones confer benefits that locked smartphones lack. They typically use a removable SIM card, or subscriber identity module, which is how some carriers know when you’re connected to the network and what services you have access to. It stores information such as your phone number, contacts, and other basic telephony, too. In theory, switching between carriers is as easy as swapping one telecom’s SIM for another.

Another advantage? Unlocked phones are sometimes cheaper, depending on the make and model, and they’re a boon for travel. You can take an unlocked iPhone 5S to Europe, slot in a pre-paid SIM card for a local network, and avoid your domestic carrier’s exorbitant roaming charges.

But just because a phone’s unlocked doesn’t mean it’ll work on your carrier. First, you need to ensure it’s compatible with your telecom’s bands. Then, you need to make sure it’ll work with your carrier’s cellular standard of choice.

Just what are bands, exactly?

Before you try to wrap your head around the idea of bands, it’s important to understand frequency. Frequency, formally defined as the number of cycles per unit time, refers to vibrations in a medium — i.e. water, air, or solid matter — rapid enough to produce a wave. Each wave is measured over the course of a second in the hertz unit, named in honor of the 19th-century German physicist Heinrich Rudolf Hertz. And the idea is relatively simple: the higher the number of vibrations, the higher the hertz.

The categories of waves are practically boundless. You’re no doubt familiar with sound waves, produced by everything from the vocal chords of a baritone singer to the strings on a violin. An “A” note on a stringed instrument vibrates at about 440 Hz, for instance, or at 440 times per second. Light, a special category of wave known as an electromagnetic wave, is another example. Visible light lies in the range 4-8×1014 Hz, where 4×1014 Hz is red light and 8×1014 Hz is violet light. Cellular is a wave, too. The invisible tether between your phone and your carrier’s towers has a frequency somewhere in the range 300 GHz to 3 kHz, low enough to pass invisibly around us.

That range may sound enormous, but frequencies are a finite resource. They’ve been commandeered by television broadcasters, radio stations, cellular carriers, government agencies, and researchers to deliver voice, video, calls, data, and more. Unfortunately,  interference is a problem and two radio stations broadcasting at the same frequency will degrade one another’s signal. That’s why in recent U.S. history, spectrum — a given range of frequencies — has been allocated by the Federal Communications Commission (FCC). The exact method of divvying it out has evolved over the years, but today, so-called “blocks” of frequency are distributed in auctions where the bids exceed billions of dollars. Verizon and AT&T have set aside more than $16 billion in the past 30 years alone.

To complicate matters even further, not all frequencies are created equal. Lower frequencies are often described as “beach-front property.” They have superior penetration, meaning they can pass through walls and other objects, and better range. That’s especially relevant in the cellular industry, where the cost of building a single tower can easily exceed $250,000. A network operating at 850 MHz needs two to four times fewer towers than one operating at 1,900 MHz.

That’s where bands come in. Generally speaking, bands — short for bandwidth — refer to the range of frequencies that can be contained within a signal.

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