Ever since the Librarian of Congress decided not to renew a provision in the Digital Millennium Copyright Act that essentially makes it illegal to unlock your phone without your carrier's permission, there's been intense interest in the issue. But when it comes to unlocked cell phones, there's also been a lot of confusion.

Many people have been looking for ways to unlock their devices. The U.K. Web site Mobile Unlocked, which sells unlock codes to consumers, says sales of unlock codes are up 71 percent. But others are still trying to figure out what the heck device-locking is all about and how and if their own smartphones can be unlocked. There's no question millions of consumers are still confused about cell phone unlocking. Aside from the legal issues, there are technical issues that may even make cell phone unlocking impossible for some consumers.

In this edition of Ask Maggie, I re-examine this issue and offer some basic information about what a cell phone lock is. I explain on which wireless networks cell phones can be unlocked. And I caution consumers to investigate before they buy a device they think is unlocked.

I also explain to another reader why he can't bring just any phone to his prepaid Virgin Mobile service.

Locked versus unlocked phones: What's it all mean?

Dear Maggie,

Could you please explain the concept of carrier locks on cell phones? It is a tremendously confusing topic. I think I know the basics, but there are always strange technicalities that I keep seeing that just throw me off and make me question whether I really know how the system works.

For context, I'm a Verizon customer with an iPhone 4S. Each summer my family visits Crane Lake, Minn., where Verizon has zero coverage but AT&T has very broad coverage. We've resorted to using crappy basic phones with prepaid AT&T calling plans for simple communication (we have no Wi-Fi/Ethernet networking for our iPhones/computers). It would be great if AT&T would let us use a prepaid plan for one of our iPhones to use for the two weeks we visit each summer, but for some reason they won't.

OK, so a lot of that info was irrelevant, but I've been scouring the Web sites of Verizon and AT&T and trying to learn more about all of this.

Here are my questions:

What are the explicit differences between a locked and unlocked phone?

Why is the iPhone treated differently by carriers relative to other phones?

How do Verizon's and AT&T's lock policies relate to each other? Is a Verizon unlock the same as an AT&T unlock?

Thanks,

Anonymous

Amanda Kooser/CNET

Dear Anonymous,

You are absolutely correct about this being a confusing topic. So let's see if I can explain some of the basics for you.

Carrier locks come with just about any cell phone you buy from a wireless operator. Even if you aren't buying the device directly from a particular operator but are buying it from a retailer, such as Amazon or Best Buy, and it's for a specific carrier, then most likely it has a carrier lock on it. This is regardless of whether you buy the phone with a subsidy and a two-year contract or if you buy the device at full price.

Generally, the only devices that do not have carrier locks are ones that specifically say they are "unlocked," such as the Google Android Nexus brand of devices, the unlocked version of the iPhone, which is sold at the Apple store, and some developer edition devices. If you want an unlocked phone, you should research in advance whether the phone you are purchasing is available as unlocked and where you can buy it as an unlocked device. Remember: Paying full price alone doesn't guarantee that a smartphone is unlocked out of the box.

What is a phone lock?

The lock is really a software code that's put on the phone by the manufacturer as per the requirement of the carrier that sells the device. And the lock is meant to ensure that the phone can't be used on any other operator's network until a different software code is entered to unlock the device.

This is an issue that's most important for devices that operate on GSM networks. This is a wireless standard that used by AT&T and T-Mobile in the U.S. and by most operators around the world, especially in Europe, Asia, Canada, and Africa, as well as parts of Latin America. The 3G technologies HSPA and HSPA+ are based on GSM, which means carriers offering HSPA or HSPA+ also operate via GSM.

All GSM devices are designed so that service is provisioned using a SIM card. With an unlocked device, a GSM smartphone can be reprovisioned and used on another network simply by popping out the old SIM card and putting in a new one from the new carrier. The carrier doesn't necessarily need to be notified, and you don't need anyone in the store to reprovision your phone.

This is not the case with phones developed for CDMA networks. This is the technology used by Verizon Wireless and Sprint in the U.S. It's also used by some carriers in Latin America, Asia, and Canada. The standard is not as widely used as GSM. CDMA devices do not have SIM cards. So if you wanted to take your CDMA device to another CDMA carrier, because a CDMA-only device can't be used on a GSM network and vice versa, you'd have to get the carrier to provision the device for that other network. EV-DO is the 3G technology used on CDMA networks.

In general, CDMA carriers don't reprovision devices made for other networks for use on their own networks. So this means phones made for Verizon, which are using the same basic technology as devices made for Sprint, won't work on Sprint's network. In other words, you can't reuse a Verizon device on Sprint and vice versa.

But unlocked GSM phones from AT&T and T-Mobile will work on each other's networks, which makes the use of GSM much more flexible than CDMA.

In summary, phone locks are not really relevant when you're talking about 2G and 3G devices that operate on CDMA or EV-DO networks. But phone locks are very important for devices that operate on a GSM or HSPA/HSPA+ network. Almost every GSM device comes "prelocked" to a particular carrier. Certain phones are sold unlocked. And if you have a device that is locked, you can get it unlocked from your wireless carrier if you meet certain criteria, which includes paying the full price of your device or ending your contract and being in good standing with your service provider.

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Complicating factors

So that's the basic gist of how software locks for smartphones work. But things are getting a bit more complicated because now there's a new network technology that's currently used only for data services but will eventually be used for voice too. That technology is called LTE. And like GSM, LTE uses a SIM card.

But unlike GSM, not all LTE services operate over the same radio frequency. For instance, to deliver their LTE services, AT&T and Verizon Wireless use different slivers of spectrum that have different "band plans." This means devices built for AT&T won't operate on Verizon's LTE network and vice versa.

This issue will soon change as chip manufacturers start including multiple radios on their semiconductors. What's more, wireless operators will also soon be incorporating other slivers of spectrum into their LTE networks, which will overlap with their competitors. When these things all start to converge, we'll likely see more interoperability among devices that include support for the faster-speed LTE services.

Meanwhile, many of the smartphones being built today for Verizon and Sprint also include GSM capability on top of CDMA and LTE. If a device also supports GSM, then it has a SIM card, which can be swapped out to access almost any GSM network.

Remember that in the U.S., Verizon and Sprint customers use either a Verizon or Sprint CDMA network. The GSM radio is included in these phones so that subscribers can roam onto networks in Europe and other parts of the world. Verizon and Sprint subscribers can choose either to sign up for international roaming plans with their U.S. carrier when they travel, or, if they can get their devices unlocked, they can swap out the SIM, put in a SIM card from a local provider, and get new service that way.

Verizon 4G LTE devices are unlocked out of the box

One thing to note here in terms of software phone locks is that all Verizon 4G LTE smartphones come unlocked out of the box. The reason why is that the spectrum Verizon is using to build its 4G LTE network had restrictions put on it by the Federal Communications Commission, which required the company to allow "open access" to the network. So as part of this provision, Verizon has decided not to lock those devices. That said, its 3G devices are locked.

So what does this mean for the average consumer? Let's take the iPhone 5 as an example. It's a 4G LTE device. A Verizon version of this phone comes unlocked out of the box. There aren't special codes that need to be entered in order to use it on another carrier's network. But because of the spectrum differences I mentioned above, a Verizon iPhone 5 won't operate on AT&T's LTE network. It will operate on AT&T's 3G network, which is based on GSM. So this means you can use the Verizon iPhone on AT&T, but you won't get the fastest Internet speeds on it.

The same is true if you take the Verizon iPhone 5 to Europe. You can swap out the SIM card and put in another carrier's SIM card, but the device will operate only on a GSM-based 3G network. And it won't operate on a faster LTE network.

And even though the phone is unlocked and uses the same CDMA technology and some of the same radio frequency channels as Sprint, an unlocked iPhone 5 won't operate on Sprint.

Answering your specific questions



What are the explicit differences between a locked and unlocked phone?

The difference between a locked and an unlocked phone is that a locked device has a software code on it that prevents you from taking a GSM-based device and using it on another GSM carrier's network. An unlocked phone either doesn't have the lock software on it or someone was able to get a code that unlocks the software. Once a device is unlocked, you can pop out the SIM card and put in a different SIM from another GSM operator and get service. Remember that this issue of locked and unlocked phones today is really only relevant when you're talking about devices made for GSM networks. It doesn't really apply to CDMA-only devices or for swapping out SIM cards for LTE networks. As I explained above, most carriers use different frequencies and band plans for their LTE networks, so even without a software lock on the device, it still won't operate on these networks. That will soon change, but for now don't expect unlocked LTE devices to perform at top speeds on any other carrier's LTE network.

Why is the iPhone treated differently by carriers relative to other phones?

In the past, the iPhone was treated differently than other devices when it came to lock codes. For instance, AT&T was happy to unlock any other phone you owned, but according to its policy, the iPhone could never be unlocked. The reason for this was because AT&T didn't want people buying the popular phone and going to another carrier, whether that was T-Mobile or an overseas operator. So to control how and where the device was used, it refused to unlock it.

That's changed. Now AT&T, T-Mobile, and Sprint let people unlock any smartphone so long as they've met the carrier's unlocking criteria. Usually, this means the phone is fully paid for and the customers are no longer on a contract. And they have to be customers in "good standing" for a certain period of time.

How do Verizon's and AT&T's lock policies relate to each other?

Is a Verizon unlock the same as an AT&T unlock? As I explained above, Verizon has a slightly different unlock policy for its newer 4G LTE phones, which was born out of an FCC requirement. The spectrum AT&T uses for its 4G LTE network and for its 3G HSPA and HSPA+ network don't have the same requirements, so every device sold by AT&T or for use on AT&T's network has a software lock on it.

Also, as I've mentioned, you can get a special code to unlock the software from AT&T so long as you meet the company's requirements. The same is true if you have a Verizon 3G device that supports GSM network technology. But if you have an LTE device from Verizon, you don't need an unlock code. The device is already unlocked.

The AT&T code is not the same as the Verizon code. So if your device is locked, you must get the appropriate unlock code from your carrier, or if you are getting the code elsewhere, you should specify which carrier network your device is affiliated with.

Another way to get an unlocked phone is to buy one that's specifically advertised as unlocked. For instance, Google's Nexus series of smartphones come unlocked. They're built and configured in such a way as to allow any GSM SIM to be used in them. The international unlocked version of the iPhone is also sold without a lock. And again it can be used on any GSM network. (This means that neither the Nexus series phones nor the unlocked iPhone can be used on Verizon or Sprint's CDMA voice or data networks.)

Your vacation dilemma

As for what you should do while you're on vacation in Minnesota, so long as your iPhone is unlocked and supports GSM, you should be able to use an AT&T SIM card in it. So use the existing SIM card you've used in the past or buy a prepaid SIM from AT&T and pop it into your unlocked iPhone.

As I said, the iPhone 5 on Verizon is already unlocked out of the box. So you should be able to use any AT&T SIM card in that device and get service. If you want to do this on the cheap, make sure you turn off all the data functions on the iPhone, and you'll just be able to use the voice network.

The iPhone 4S is not a 4G LTE device, so if you own this device from Verizon, you'll have to get an unlock code from Verizon in order to unlock it. I would call a Verizon customer service agent and tell her you're going out of the country on vacation and you want to use a local SIM card. She'll tell you the requirements for getting your phone unlocked. And if you have met the appropriate criteria, the agent should provide you with the code and instructions for unlocking.

I hope I answered your questions and helped you better understand the differences between locked and unlocked devices.

Good luck!

Can I bring a Samsung GS4 to Virgin Mobile?

Dear Maggie,

Virgin Mobile is not offering the Samsung Galaxy S4 yet. Will I have a problem connecting/using a GS4 if I buy it from the secondary market?

Thanks,

Mike

Dear Mike,

I'm sorry to tell you, but the answer to your question is yes. Virgin Mobile is a prepaid brand owned and operated by Sprint. And Sprint is a CDMA wireless operator. As I explained as part of my above answer to the question about locked and unlocked phones, CDMA devices don't have the same flexibility in terms of device portability as GSM phones.

Sarah Tew/CNET

So even if you buy an "unlocked" GS4 on the secondary market, it will work only on a GSM network. It won't work on Virgin's CDMA network. The main reason is because that unlocked GS4 doesn't have a CDMA radio that will connect to the Virgin network.

Sadly, even if you were able to get your hands on a Sprint Samsung GS4, it still wouldn't work on Virgin's network. This is actually something that as a consumer I find maddening. As I said above, Virgin is owned and operated by Sprint. It uses Sprint's 3G wireless network that's based on CDMA, so technically there's no reason why a phone made for Sprint shouldn't work on Virgin's network.

But because of software locks and how the phones are provisioned for each network, Virgin Mobile customers are restricted from using Sprint devices on a Virgin service. Sprint launched a new program in March that lets customers use their old Sprint devices on other mobile virtual network operators that use the Sprint network, but the program does not work for Virgin Mobile customers.

There is a chance that you may find a tech specialist who may offer to unlock a Sprint so it works on Virgin, but a Sprint spokeswoman cautioned the phone may not function completely on Virgin's network.

"For example, voice services may function," she said in an email. "Although many other features and services may not and system and network updates may become problematic over time."

Boost is another prepaid brand owned by Sprint. Like Virgin Mobile, most of Sprint's phones won't work on this network. But there are select Sprint devices that can be activated on Boost's network. The normal reactivation fee associated with Boost used phones will apply, the spokeswoman said.

I hope this was helpful. Good luck.

CORRECTION August 19, 2013:A previous version of this article incorrectly stated that Virgin Mobile and Boost Mobile participate in Sprint's Bring Your Own Device program for MVNOs. Virgin Mobile and Boost Mobile are wholly owned subsidiaries of Sprint. Sprint does not allow its used devices to be used on Virgin Mobile. Only select Sprint devices can be used on Boost Mobile. The story has been corrected

Ask Maggie is an advice column that answers readers' wireless and broadband questions. The column now appears twice a week on CNET offering readers a double dosage of Ask Maggie's advice. 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.