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 Luckily, that interpretation was superseded by the Unlocking Consumer Choice and Wireless Competition Act of 2013, which recognized unlocking as an exception to intellectual property protections.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.

Cellular standards

Questions around bands usually become pertinent when you’re deciding which phone to buy off contract. First things first — you have to figure out which network standard it uses.

You’ve probably heard of 2G, 3G, 4G, and you might even be aware that they refer to technologies in use by carriers such as Verizon, AT&T, T-Mobile, and Sprint. Each is defined by the International Telecommunications Union, the governing telecommunications body that decides on cellular standards.

In the U.S., two technologies dominate: GSM (Global System for Mobiles) and CDMA (Code Division Multiple Access). They’re two standards that wirelessly transmit voice and data to and from your phone, but they’re completely incompatible. No amount of finagling will get a GSM phone to work on a CDMA network, and the opposite is just as true. In the U.S., T-Mobile and AT&T use GSM, and it’s by far the most popular standard globally. CDMA is used by Verizon and Sprint, but it’s largely a dying breed.

That said, there’s a third class of phone that’s compatible with both the CDMA and GSM standard. Many manufacturers produce unlocked handsets that are compatible with a wide range of standards. If, for example, you buy an unlocked iPhone 7 direct from Apple or an unlocked Pixel XL direct from Google, it will work with both standards. A few other examples include Apple’s iPhone 6S, 6S Plus, 6, and 6 Plus, along with Lenovo’s Moto X Pure Edition and Google’s Nexus 6, 5, 5X, and 6P.

If you’re buying through a third-party retailer, then make sure you have the correct model number for the phone, and double check with the manufacturer about the bands it supports. Not all GSM and CDMA carriers are created equal. Even though Sprint and Verizon employ the same cell standard, their implementations differ. Thanks to the enduring race for spectrum, Verizon transmits data and voice at different frequencies than Sprint. The result? A phone compliant with CDMA might work on Sprint but not on Verizon, or only partially on Verizon. That might manifest as cellular reception near some cell sites but not others, or voice and texting capabilities but no data, or slower-than-average internet speeds.

How to check if your phone is compatible

When it comes to network compatibility, 3G and 4G are of principle concern. That’s because technologies that fall under 2G’s umbrella are slowly on the way out. AT&T has pledged to shut down its 2G network by sometime next year. Verizon aims to retire 2G in 2019. T-Mobile and Sprint are expected to follow suit.

In terms of 3G technologies, there are several to consider: Universal Mobile Telecommunications Systems (UMTS) and High-Speed Packet Access (HSPA+). The latter’s an improved flavor of the former, and, as such, operates on the same frequencies. The five most common UMTS 3G frequencies around the world are 800, 900, 1700 (AWS), 1900, and 2100.

The easiest way to check if the phone you’re considering will work with your carrier is by using an online database like WillMyPhoneWork. Failing that, though, there’s the manual method: cross-referencing your telecom’s 3G and 4G bands with the bands that your unlocked smartphone supports. As long as the two columns match up, it’ll be fully compatible.

Here’s a chart that includes the 3G network technologies, bands, and frequencies of all five major U.S. carriers:

Carrier Network 3G Bands 3G Frequencies
AT&T GSM/UMTS/HSPA+ 2, 5 1900, 850
Verizon CDMA 0, 1 850, 1900
T-Mobile GSM/UMTS/HSPA+ 2, 4 1900, 1700/2100
Sprint CDMA 10, 1 800, 1900
US Cellular CDMA 0, 1 850, 1900

Complicating matters further is a third standard: LTE. Short for Long-Term Evolution, it boasts a number of improvements over older CDMA and GSM technologies. Uploads and downloads are much faster, and capacity’s higher. Generally speaking, call quality (Voice over LTE, or VoLTE) is improved, too.

But phones compatible with one carrier’s LTE network won’t necessarily work on another telecom’s LTE network. That’s because LTE is an umbrella term for variants. There are more than 40 different LTE frequency bands used around the world. In the U.S., Sprint uses LTE (TD-LTE), a version of LTE with relatively low compatibility. Sprint, T-Mobile, Verizon, and AT&T all have LTE bands of their own.

Here’s a chart that includes the 4G bands and frequencies of all five major U.S. carriers:

Carrier 4G LTE Bands 4G LTE Frequencies
AT&T 2, 4, 12, 17 1900, 1700 abcdef, 700 bc
Verizon 2, 4, 13 1900, 1700 f, 700 c
T-Mobile 2, 4, 12 1900, 1700 def, 700 a
Spring 25, 26, 41 1900 g, 850, 2500
US Cellular 5, 12 850,  700 ab

MVNO

If you’re on a mobile virtual network operator, or MVNO, the process of sussing out your smartphone’s compatibility is the same … mostly. Carriers such as Metro PCS and Cricket sell service atop larger telecoms, piggybacking on networks like T-Mobile and Verizon. Once you identify which network your MVNO uses to deliver service, referring to that parent network’s frequencies and bands is the safest way to determine whether your unlocked handset is compatible.

Setting up your unlocked phone

With knowledge of your carrier’s bands in hand, you resume smartphone comparison shopping.

Once the unlocked phone’s in your possession, the next step is configuring your phone to work with your network. Some smartphones perform an automatic setup when you pop in a SIM card, but others require a bit of manual configuration. You’ll need to make sure the APN, or access point name, is configured according to your carrier’s settings.

On an Android phone, the setup process is relatively straightforward:

  1. Navigate to the settings menu.
  2. Tap Mobile networks.
  3. Tap Access Point Names.
  4. Tap the Menu button.
  5. Tap New APN.
  6. Enter the values provided by your carrier.
  7. Restart the device.

The process is about the same on the iPhone:

  1. Tap Settings.
  2. Disable Wi-Fi.
  3. Tap Cellular and make sure that Cellular Data is enabled.
  4. Tap Cellular Data Options and make sure that LTE/3G Data is enabled.
  5. Tap Cellular Data Network.
  6. Enter the values provided by your carrier.
  7. Restart the device.

You should begin to receive service normally. If you don’t, you may have to contact your carrier about your International Mobile Station Equipment Identity (IMEI). Some carriers use it to restrict LTE service based on the number associated with your account. That’s not the case with GSM — GSM carriers don’t have a mechanism for determining your phone’s IMEI number — but if you want 4G speeds, you’ll have to get in touch with a customer service representative who can register your device with the appropriate plan.

Failing that, you can provide the IMEI number of a carrier-branded phone. Or you can set up your account on a carrier-branded phone first, and then switch the SIM card into the unlocked phone.

If it all sounds unnecessarily complicated, that’s because it is. Thankfully, smartphones that support a wider range of bands are growing in popularity, so it should be less of a headache to figure out in years to come. In the meantime, refer to our section on how to check your phone is compatible.

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