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Switching carriers? You may be able to take your iPhone 5S with you

Will my Verizon iPhone 5S work on AT&T? That's a question asked by one Ask Maggie reader confused by the puzzling state of device interoperability. This week's column tackles the issue.

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One of the most frustrating things about switching wireless carriers is having to buy a new smartphone. At a full retail price of between $600 and $650, a smartphone is an investment for many consumers.


For years, it was standard practice that when you started service with a new mobile operator, you'd have to buy a phone specifically designed for that carrier. It was particularly true when switching from a carrier like AT&T to Verizon, and vice versa.

But those days may soon be coming to an end, as wireless operators start using the same network technology as well as common radio frequencies to build their next-generation networks. And the movement could gain momentum as handset makers, looking to reduce costs, limit the number of product models they manufacture by packing them with multiple technologies and wireless radios that can be used across a variety of operators.

Taking your device with you when you switch carriers is likely to become more important to consumers, as more wireless providers provide financial incentives, like lower service costs, for subscribers who bring their own devices to their networks. What's more, the new cell phone unlock bill that was recently signed into law by President Obama highlights the anticompetitive nature of forcing consumers to buy a new device when they switch carriers.

In this edition of Ask Maggie, I help a reader figure out if he can take his coveted iPhone 5S with him when he makes the switch from Verizon to AT&T.

The big switch

Dear Maggie,

I was thinking of switching from Verizon to AT&T this fall, but I keep getting conflicting reports on whether my current Verizon iPhone 5S will work on AT&T's network. AT&T says it won't be a problem; Verizon says it's impossible. Also, I bought the phone at a subsidized price of $200 with a two-year contract. Who's telling the truth?


Dear Dave,

The short answer to your question is that the AT&T representative is correct. You should be able to use your iPhone 5S from Verizon without any problems on AT&T's network. The same goes for using it on T-Mobile's network.

All you have to do is pop out the SIM card and replace it with one from AT&T. The phone will not only be able to operate over AT&T's GSM-based voice and text messaging network, but you should also be able to get 4G LTE speeds on the phone.

Apple's iPhone 5S
Apple's iPhone 5S Sarah Tew/CNE

The fact that the phone is still under contract shouldn't matter in terms of taking it to AT&T. But as I will explain later, this is only the case because you're taking a 4G LTE device from Verizon to another carrier. (All Verizon 4G LTE devices are unlocked out of the box.) That said, you may still be required to pay an early-termination fee if you leave Verizon before your contract ends.

Now let me explain why there may be some confusion about whether the Verizon iPhone 5S will actually work on AT&T's network. And I'll also provide insight on whether iPhone 5S handsets from other carriers are interoperable with other US carrier networks.

What's needed for device interoperability?

First, let's start with the basics. I'm sure you realize that for years most cell phones weren't interoperable among all the US carriers. There's always been a big divide between AT&T and T-Mobile devices, which for the most part have always been interoperable, and devices from Verizon Wireless and Sprint.

The reason is that AT&T and T-Mobile use a standard known as GSM. This technology is used throughout much of the world and relies on SIM cards, which can be swapped in and out of unlocked devices to provision service.

Meanwhile, Verizon and Sprint use a standard known as CDMA. Devices using this technology don't have a SIM card. This means the carriers have to provision the service for you. So for the most part, even though Verizon and Sprint devices use the same technology and often use the same spectrum, they can't be used on each other's networks.

In the era of next-generation 4G wireless networks, all the major mobile carriers throughout the world, including the four major operators in the US, are finally using the same technology, known as LTE. But unlike with the older GSM and CDMA technologies used to deliver 2G and 3G services, carriers have been deploying this new technology using different wireless spectrum frequencies.

This is important, because not only must the wireless technology be the same in order to have device interoperability among different wireless operators, but the spectrum frequencies and the so-called band plans used to define those spectrum frequencies in devices need to be the same too.

In the early days of LTE deployments, the frequencies used by US operators were all different. Verizon had a nationwide license for 700MHz spectrum that only it had access to, and the band class used in its LTE devices was used only by Verizon. This meant early Verizon LTE devices weren't interoperable with LTE networks developed by other carriers, such as AT&T, which had deployed service using similar spectrum that used a different band class for its devices.

Interoperability is coming

Luckily, for consumers that's all changing. As Verizon and other wireless operators expand their networks, they're starting to use common slivers of wireless frequency to deploy their service. For example, Verizon is now using what's called AWS spectrum to expand its 4G network. And because it's using this spectrum, its devices must support LTE band 4. AT&T and T-Mobile also use AWS spectrum and their devices also support LTE band 4.

By contrast, Sprint is not using any radio frequencies that match up with its competitors in the US. This means that when it comes to LTE, its devices may not include the same radio frequencies and LTE bands as its competitors.

That said, one thing to keep in mind is that smartphone manufacturers are in a volume business. This means they'd rather build one device that covers several carriers rather than build custom hardware for specific carriers. And for a carrier like Sprint, which is less than half the size of a Verizon, this is even more true.

What's more, Verizon and Sprint each realized years ago that being CDMA carriers in a GSM world was frustrating for their subscribers, so they started offering so-called "world phones," which offer CDMA for domestic voice, text and 3G data, as well as GSM technology for foreign roaming. Because GSM and now LTE use SIM cards for service provisioning, devices for Verizon and Sprint also include SIM slots. Pure CDMA devices from these carriers don't include SIM slots and must be provisioned by the carrier.

From a consumer's perspective, this is good news. Today, most, if not all, 4G LTE devices from Verizon and Sprint are technically capable of operating on a GSM voice and text messaging network, as well as operating on a GSM-based 3G data network. Though the functionality was included to allow roaming on foreign GSM networks, it also means these devices can be used on AT&T's and T-Mobile's 2G and 3G networks.

When it comes to LTE, depending on which LTE frequencies and band classes are supported on the device, Verizon and Sprint phones could also be interoperable with their GSM competitors' LTE networks.

Different versions of the iPhone 5S

The iPhone 5S is a great example of a device that packs in multiple technologies, radio frequencies and band classes, making its hardware interoperable with several carrier networks. For Apple, it was important from a manufacturing and cost perspective to have as few models of the iPhone as possible. The company also included many of the same radios and components in the different versions of the phone it built for different markets.

As a result, every version of the iPhone 5S supports basic GSM-based technologies for 2G and 3G services. This means every iPhone 5S sold around the world can at least roam onto a GSM carrier's voice, text messaging and 3G data network.

Things start to differ when it comes to LTE and CDMA. The four major US wireless operators are using three different versions of the iPhone 5S. AT&T and T-Mobile are selling the GSM model (A1533); Sprint is selling the CDMA model (A1453); and Verizon Wireless is selling the CDMA model (A1533).

The AT&T and T-Mobile versions of the iPhone 5S are identical. And the Verizon Wireless version of the iPhone 5S is also the same as the AT&T and T-Mobile versions. The only big difference is that the Verizon version also includes CDMA support. And the AT&T and T-Mobile GSM versions do not.

This table was assembled with information from Apple regarding the different versions of the iPhone 5S made for the US market. CNET/Marguerite Reardon

Sprint's version of the iPhone 5S doesn't support all the same frequencies as the iPhone 5S made for AT&T and T-Mobile or the one made for Verizon. But if you look at the chart above, you'll see that some of the supported frequencies and band classes overlap.

What this means for consumers is that from a hardware perspective, an iPhone 5S made for Verizon or Sprint could operate on AT&T's and T-Mobile's networks. Why? The Verizon and Sprint versions both support LTE bands 2, 4 and 17. LTE bands 2 and 4 are used by both AT&T and T-Mobile. And Band 17 is used by AT&T.

Because the Verizon and Sprint devices also include GSM, they could technically get 2G and 3G voice and data services on either AT&T's or T-Mobile's networks.

Unfortunately, the same is not true in reverse. While iPhones from AT&T and T-Mobile could operate on Verizon's LTE network because the devices all include support for LTE bands 4 and 13, which are used by Verizon for its LTE service, they don't support CDMA at all. So wireless customers wouldn't be able to access Verizon's voice or 3G data network.

This table offers information regarding the 4G spectrum frequencies and LTE bands supported by the four major US wireless operators. CNET/Marguerite Reardon

With Sprint, things are bit trickier. Since Sprint is using less-common spectrum frequencies to build its LTE network, the LTE bands on the AT&T, T-Mobile and even Verizon iPhone 5S devices don't match up as well. In fact, these devices support only one LTE band that's used by Sprint.

What about carrier device locks?

Now that I've explained why it's technically possible for any of the iPhone 5S smartphones to work on any other carrier network in the US, when it comes to LTE, I'll also explain why the reality is actually quite different.

In addition, to there being technical limitations when it comes to device interoperability, there's also another barrier: carrier software locks. This is additional software that's been added to the devices at the carrier's request, which locks the device to a particular carrier. The locks can be lifted with special codes you can get from the carrier. A new law signed by President Obama dictates that carriers are required to provide these unlock codes to customers who own their phones and are out of contract. But if you are still within your 2-year contract, the carriers don't have to unlock your phone. And some, like Sprint, have very strict policies about this.

Verizon is a special case

While most devices sold through a wireless carrier have these software locks on them, 4G LTE devices sold through Verizon Wireless do not. Why? One of the Federal Communications Commission's conditions put on the spectrum Verizon originally used to build its 4G LTE network requires "network openness." Verizon has interpreted this to mean that the devices it sells for use on this network must be open, which means no lock. What this means for you and other Verizon customers with 4G LTE devices is that there is no software lock on these smartphones. They come unlocked out of the box.

The bottom line

As a Verizon customer moving to AT&T, you're in a great position to reuse your iPhone 5S. You should be able to pop out the SIM card and replace it with a SIM card from AT&T or T-Mobile, turn off CDMA roaming, and you should be able to get AT&T 2G, 3G and even 4G LTE service.

The key to figuring out if a device will operate on another carrier's network is to look at the device specification sheet. What you're looking for here are the network technologies and radio frequencies, as well as band classes supported on the device. Then you need to understand which technologies, radio frequencies, and LTE bands that carrier is using on its network.

If the device from carrier A includes technology and radio support that matches those of carrier B, you should be good to go from a hardware perspective. After that, you'll need to figure out if the device from carrier A is already unlocked, and if it's not, you'll have to get it unlocked before you can use it on carrier B's network.

Once the device is unlocked and you've determined that the device from carrier A matches the requirements of carrier B's network, your old device should work just fine.

I know this is a very confusing topic. I hope this explanation was helpful. Good luck with the switch from Verizon to AT&T.

Ask Maggie is an advice column that answers readers' wireless and broadband questions. If you have a question, I'd love to hear from you. Please send me an e-mail at maggie dot reardon at cbs dot com. And please put "Ask Maggie" in the subject header. You can also follow me on Facebook on my Ask Maggie page.