The subtle danger of unlocked smartphones: Network compatibility


With the Big Four wireless operators in the US having switched away from subsidized smartphone plans, 'unlocked' devices have been trending up—even more so than in the past. Unlocked smartphones come in many shapes and forms, meaning nowadays you can grab one from Apple or Samsung, but also import a unit or two from overseas. Like China. And that can be a problem.

It can be a problem, because beyond the technical specifications of the device in terms of processor, display, and camera, there are also network constraints to consider. And even though many Chinese makers have been improving in this regard and now push out smartphones that will work on US LTE networks, your mileage may vary. It may vary because of...

LTE frequency bands 


Today, you can stream a gigantic, 4K UHD movie on your phone using your LTE connection. That's incredible—a decade ago that would have been unthinkable. But that LTE connection operates within the rigid world of physics, and in this case 'spectrum'. In simple terms, all networks beam signal from their towers at a given frequency, typically denoted by a mega-hertz.

For example, Verizon supports LTE bands 2, 4, and 13. That's the easy way to refer to them, though these denote the 1900MHz, 1700MHz, and 700MHz frequencies if we're being more specific. You can get by with a device that supports just one or two of these in order to get LTE on Verzion's network, and many people consider this a green light to import that dank Elephone, but keep in mind that this will almost inevitably lead to a subpar experience.

Lower versus Higher frequency bands



Even if you're not a techie, you might have heard it said that lower frequency bands (i.e. 700Mhz) are somehow inherently superior to higher frequency ones (say, 1900MHz). That's true, but only to an extent.

The lower the frequency, the easier it is for a single cell tower to propagate signal. This means that one 700MHz tower, all being equal, can outperform a 1900MHz one in terms of coverage, as well as penetration. That is, lower frequency bands are better able to make their way through walls and such, and suffer lower attenuation. So if you live in an urban area, these towers are ideal, and they're also pretty popular in rural areas where coverage is more of an issue compared to capacity (or speeds).

While the above is a fact, higher frequency towers are usually enmeshed with lower frequency ones in order to improve the capacity of a carrier's network, specifically in metro areas where every subscriber is fighting for bandwidth. These are more numerous because of their shorter range, but this also means they provide higher capacity.

In short, then, you want a smartphone that can utilize both high and low bands for optimal performance. Otherwise, you're running the risk of being stuck with a congested tower in the city, or lack reception altogether when in the wild (relatively speaking, don't expect to get reception everywhere). 

Carrier certification


The above was a very simplistic look at the role frequency bands play into overall network performance. In reality, an unlocked device, especially one imported from abroad, is almost assured to perform relatively worse than one that is sold directly by the carrier, all things being equal. That's because, while we love to hate on carriers for their slow and painful (from manufacturers' perspective) certification procedures, therefore slowing down the rate at which we get software updates, these are there for a reason.

Nobody understands a carrier's network better than... you guessed it, the carrier itself. Apart from the basic criteria for eligibility (i.e. the aforementioned support for bands), carriers also further optimize devices for better performance on their networks. You probably don't really consider this aspect of the performance equation, but even things such as elevation and atmospheric conditions can affect your reception. A carrier-certified device will usually balance these out so you get the best possible experience—it is in their interest, after all.

Put simply, even with identical hardware and software, an unlocked smartphone will almost surely offer worse cellular performance than one from a carrier. Keep that in mind if you're easy to set off by poor coverage or live in a problematic area.

Real-world examples


We mentioned Sony, which is a popular maker that regularly doesn't partner with US carriers for its releases and instead sells its devices through its own website and third-parties such as Amazon, Best Buy, B&H Photo, and so on. Say you dig the latest Sony Xperia X or Xperia X Performance, for example. 

Apart from not being network certified, these two also have no support for bands 13 and 25, but cover bands 2, 4, 12, and 17 among others. Using the cheat sheet table I stole from Victor's piece, we can surmise that these two Sony high-ends support all of AT&T's and T-Mobile's bands, but have spots for Verizon and Sprint. In result, performance on the latter two's networks will not be ideal.


Carrier4G LTE BandsFrequencies
AT&T2, 4, 5, 171900, 1700 abcde, 700 bc
Verizon Wireless2, 4, 131900, 1700 f, 700 c
T-Mobile2, 4, 121900, 1700 def, 700 a
Sprint25, 26, 411900 g, 850, 2500
Europe3, 7, 201800, 2600, 800
China, India40, 412300, 2500
*main band for each carrier is marked in bold

Still, the above scenario is fairly palatable. If you're the more exotic type, however, and are looking to import something like the Xiaomi Mi 5, there's trouble to be had. Not only is the Mi 5 a phone intended for the Chinese market, which uses a different type of LTE tech, but the two bands it supports for Western networks (band 1 and band 3) are incompatible with all of the major networks, and therefore the MVNO's that make use of that infrastructure. 

Another hot example is the Xiaomi Redmi Note 3, which offers support for bands 1, 3, and 7, all of which, again, are useless for a user in the US who's looking for an LTE-capable smartphone.

So are unlocked smartphones a bad thing?


Absolutely not! This piece isn't a stab at the unlocked market, but merely a reminder that is often overlooked from what I've seen during my years serving as the unwitting phone expert in my circles. These same distinctions are valid outside of the US, too, though Europe has, over the years, been a much more important unlocked market for overseas manufacturers (mainly from China), and therefore enjoy better support.

This also has nothing to do with unlocked devices that you can grab directly from, say, Apple. That said, if you buy an unlocked iPhone, you still need to think about the network you're planning on using it on, as there exist different models of the device, compatible with different networks. For example, an iPhone supporting the GSM network of AT&T and T-Mobile will be incompatible with that of Verizon or Sprint, both of which rely on CDMA infrastructure.

So, in a nutshell, unlocked phones are just phones, but it's best you approach such purchases with this knowledge in hand.

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51 Comments

1. FlySheikh

Posts: 444; Member since: Oct 02, 2015

Hmmm..

2. ibend

Posts: 6747; Member since: Sep 30, 2014

13. Chris.P

Posts: 567; Member since: Jun 27, 2013

Wasn't aware of AA's piece on the topic. Good stuff, though. As I said, this is no stab at unlocked devices, but rather a reminder. There are obviously plenty of good reasons TO go for unlocked.

24. TheOracle1

Posts: 2340; Member since: May 04, 2015

Hmmm. Not keeping abreast of the competition? Sounds like you and your colleagues need to check out more sites that have a better handle on this subject. Whilst I personally appreciate these articles you guys are clearly not very familiar with "foreign" phone models.

3. darkkjedii

Posts: 31529; Member since: Feb 05, 2011

I learned my lesson the hard way, when I bought the international version of the HTC Hero in 2009. An absolutely beautiful phone, that only supported 2G bands in the USA. $600 spent, and finally sold at substantial loss. That was the last international phone I bought, and the last one I ever will buy. USA carrier branded only phones only.

5. buccob

Posts: 2978; Member since: Jun 19, 2012

You should have made a proper research before dropping that amount of money. Maybe in my County we are used to buy from abroad, but it is just common sense... In our case we are more incline to NOT buy from carriers

12. TechieXP1969

Posts: 14967; Member since: Sep 25, 2013

It will come down to the brand. I have purchase 3 Samsung International versions of Galaxy devices so far, and I have no issue with shoving an ATT or T-Mobile SIM for service. They've worked perfectly. Back in 2009 when most carriers were 2G, I don't see where it would have been an issue. Even though a phone will say it can use a certain band, that doesn't mean there 700mhz band is the same as the one in the USA. Their 1900mhz band isn't the same as it is here. Verizon's 1900 on CDMA is not the same as ATT's on GSM. So a VZW phone will never work on ATT with exception of the iPhone because it has a modem that will work on both because Apple chose to make it so they just make one phone model that supports the 4 major carriers.

6. gersont1000

Posts: 473; Member since: Mar 13, 2012

I had a similar situation that made me decide to try to avoid unlocked phones. I sold my Xperia Z3 and upgraded to the European Z5. It had all the T-Mobile bands and had VOLTE, so to me this meant that I would have no signal problems. However, I immediately had many headaches because even though it had VOLTE, it was not certified to use VOLTE on T-Mobile. Whenever a call would come in, the phone would basically drop my data, pause like the phone was frozen, and then MAYBE pick up the call using a 2G or 3G signal. Luckily I was able to sell it online for not too much of a loss (and the poor guy that bought it later realized that he couldn't use it on Verizon but I wouldn't accept a return because that was his responsibility to research before buying). So now I'm spending more on my phones but I can feel safe that I'll always have the best signal and reception possible. So keep in mind that you also have to look into features you carrier supports, like TMobile's VOLTE and WiFi calling.

16. darkkjedii

Posts: 31529; Member since: Feb 05, 2011

Yeah bro, it was a headache lol. Couldn't text pics or vids, lesson learned. I made a little money back, but all and all it wasn't a good experience. Won't happen again though.

4. buccob

Posts: 2978; Member since: Jun 19, 2012

I work on devices certification for networks on my country and while carrier will require certain aspects of the phone to be configured specially for them, network performance for equal hardware will not be better for certified devices... In fact, depending on the carrier you will most likely lose features. For example AT&T used to require devices to come without wifi personal hotshot option. So that they could push their own hotspot app and charge separately...

7. TheOracle1

Posts: 2340; Member since: May 04, 2015

A few things to point out from this article. The author must be brainwashed on the need for Network Certification because 90% of carriers worldwide don't even bother with that crap. It's a fallacy that's been propagated by US carriers and become accepted somehow. A myth that's now been exposed by T-Mobile and is hurting the other carriers. You may not get all the features like VOLTE if your phone doesn't support band 12 on T-Mobile for example but everything will work just as well. I bought my Asus Zenfone 2 from Taiwan last year, popped my T-Mobile sim in and it worked perfectly with 4G LTE from the start. Secondly, the Xiaomi Redmi Note 3 Pro (that's the Snapdragon version) supports Band 4 for both LTE and 3G on T-Mobile USA. The Xiaomi Mi5 is perfectly compatible with 3G services on both T-Mobile and AT&T which is still very quick anyway. Thirdly, most phones are primarily GSM with just a few supporting CDMA which Sprint and Verizon use. So LTE questions are irrelevant if the phone itself isn't CDMA compatible. Fourth, Verizon is the main propagator if the Certification bullsh*t to force people to buy their phones. A couple of years ago I took a Nubia Z7 Max to them which was compatible with their network and it wouldn't work because it wasn't "Certified". Hogwash. Last thing, you schmucks at iPhonearena need to do more research on your articles when it comes to foreign (especially Chinese) phones.

9. Chris.P

Posts: 567; Member since: Jun 27, 2013

Slow down there tiger. I never said a phone NEEDS to be certified in order to work with various networks. I said it's certified to work flawlessly if it's sold directly by it, and that all things being equal, certification has its advantages in terms of making the best use of the infrastructure. On your second point, I never mentioned the Pro version, and Band 4 is obviously insufficient as per the article. Re-read. Mi5 is compatible with 3G, sure, but who said it wasn't?

14. TheOracle1

Posts: 2340; Member since: May 04, 2015

Your certification/optimization argument doesn't hold water bro. I import all my phones because I need dual-sim and there's zero difference in performance. I suppose a US Snapdragon Note 5 is going to outperform an Exynos Note 5 based on your logic because some network guru blessed it? Well it doesn't. Same with my Taiwanese Zenfone 2 and my son's US Zenfone 2. No difference at all on T-Mobile. I have plenty of hard earned ($) experience on this subject. Why mention the "Hot" Redmi version then? Because the only "hot" one for international buyers is the Pro version. And guess what? I own one and it's on T-Mobile USA! Your article makes it sound like it's totally useless. It's the inference you made.

17. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

IIRC, there was no Snapdragon version of the Note5, which is funny because you picked the only Note series phone that is that way.

22. TheOracle1

Posts: 2340; Member since: May 04, 2015

You're right about that. Ok replace it with the Note 4 or whatever you wish but you get my point.

51. g2a5b0e unregistered

Not true. There was no Snapdragon Note 2 either.

8. TheOracle1

Posts: 2340; Member since: May 04, 2015

Xiaomi Redmi Note 3:http://www.gsmarena.com/xiaomi_redmi_note_3-7863.php Xiaomi Mi5:http://www.gsmarena.com/xiaomi_mi_5-6948.php Your example of the Sony models is way off the mark too as you contradict yourself saying it doesn't support band 13 then saying (and showing) it does. The only LTE band it doesn't support is band 25. But guess what? It doesn't support CDMA either! Sony Xperia X Performance:http://www.gsmarena.com/sony_xperia_x_performance-7949.php

11. Chris.P

Posts: 567; Member since: Jun 27, 2013

Re-read article. Deep breaths also recommended! :)

15. TheOracle1

Posts: 2340; Member since: May 04, 2015

"Apart from not being network certified, these two also have no support for bands 13 and 25, but cover bands 2, 4, 12, and 17 among others." What did I miss there? The X Performance covers band 13. You're just plain wrong. And please stop editing your article to fit your arguments. Your "borrowed" chart even says it supports band 13. "Deep research also recommended! :)

10. TechieXP1969

Posts: 14967; Member since: Sep 25, 2013

The fact is, in the US; there is no benefit to buying an unlocked phone unless you use ATT or T-Mobile. With the exception of the iPhone, other OEM's don't place a modem in their device that if unlocked will work on all carriers. I am sure this is at the carriers request. VZW is especially and Sprint too. Their phones unlocked still won't work on a GSM network. Unless you can find a shop that can reprogram the phone, which is an option is you know where to look and it is perfectly legal because the phone is unlocked and is yours.

18. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

You're wrong on Verizon devices. Almost all flagship devices are final devices, and carry a majority of GSM bands that will work on T-Mobile's and AT&T's networks. Not all, but most. Besides that, you're talking about GSM Unlocked models. There are also phones considered universally unlocked, like the last few years of iPhones, the Nexus 5x, 6, and 6p (and supposedly the 2016 Nexus devices), the Moto X Pure 2015, and the unlocked Galaxy S7 and S7 Edge released earlier this year. Those phones carry all bands for use on the 4 major carriers in the US, both CDMA and GSM. I know this because I own a Nexus 6p and is rated to work on Verizon, AT&T, Sprint, and T-Mobile. There is no reprogramming necessary, that how they come from the OEMs. The benefits of this is updates without carrier involvement which should mean you device will be more up to date, and the ability to switch carriers without having to buy a new device.

23. TheOracle1

Posts: 2340; Member since: May 04, 2015

Doesn't matter anyway, if it's not on Verizon's "certified" list then it won't work. T-Mobile has changed the game completely in that regard. One untapped segment that they virtually own is the tourist segment, at least in South Florida. I don't know how many visitors come to the US annually but it's a significant number and not many use CDMA phones. That's some nice revenue right there.

26. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

First, as to the certification and whitelisting that you've been calling hagwash, you're both correct and incorrect. Would most of those phones with compatible bands work? Sure, they probably would, and once they move to full LTE (no CDMA at all) that's probably how it will work. But at this point, every device has to be added to their whitelist so long as they still have CDMA with registered IMEI numbers. Both T-Mobile and AT&T don't use that setup, which is why you can also take a Verizon global phone and use it on AT&T and T-Mobile. But it is that way for both Sprint and Verizon, and it always has been. Once they move to LTE only, that goes away. Secondly I was simply pointing out that he was referring to GSM unlocked devices rather than universally unlocked devices. Regardless of whether Verizon whitelists them or not, there is a difference between an unlocked device that only has GSM bands and one that has both CDMA and GSM bands.

27. TheOracle1

Posts: 2340; Member since: May 04, 2015

Switching over to lte only won't matter. The phone will still need either gsm or cdma to work as I understand it. LTE doesn't work exclusive of them. The bottom line is that Verizon and Sprint phones (apart from universal ones) simply don't work on the majority of worldwide networks. CDMA is not the world standard and just because Verizon are the biggest and baddest in the US it means nothing elsewhere. Verizon certification is simply a backhanded way to force you to buy their phones. AT&T, T-Mobile and their mvno's work the same as the rest of the world. Sprint and Verizon don't. VHS vs Beta kind of thing.

28. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

What are you talking about? At some point in the near future, there are and by all domestic carriers to drop all other cellular technologies other than LTE. Verizon and Sprint are dropping CDMA, and AT&T and T-Mobile are dropping GSM. At that point, all carriers will be running late, the only difference being the bands they operate on. Ltd doesn't currently work exclusively of CDMA or GSM, but they will when they drop them. Look it up. IIRC, Verizon plans to eliminate their CDMA network and go to only LTE by between 2019 and 2021. If those phones won't work without CDMA, then how the hell would they work at all? LTE supports voice and video communication, SMS and MMS, and data. If you're in a strong LTE area you can set your phone to only connect to LTE and be able to use all those features. There is no world standard, because CDMA is also used in parts of Asia. The MAJORITY of the world ran on GSM, but a standard implies everyone is using it, which isn't the case. Besides GSM and CDMA, there's also PCS, iDEN, and a few other technologies as well. Regardless, all carriers are moving to a truly global standard cellular technology in LTE. At that point, there should be no reason for this to occur anymore.

29. TheOracle1

Posts: 2340; Member since: May 04, 2015

Sorry, I wasn't clear. This is what I'm referring to: "Verizon has said it aims to start selling LTE-only phones in 2015, but for now, those will require special Verizon software to make voice calls, so that move won't make it any easier to switch carriers with your phone. Even without CDMA, the CDMA philosophy of carrier control of your phone will remain intact." From PCmag Feb. 2015 So basically the Evil Empire will continue to screw consumers. Another quote from the same article which applies worldwide: "If you want to switch phones often, use your phone in Europe, or use imported phones, just go with GSM. Otherwise, pick your carrier based on coverage and call quality in your area and assume you'll probably need a new phone if you switch carriers." True there is no Global standard but GSM is the overwhelming standard used worldwide. Therefore buying a GSM phone is solid advice and using it on a carrier that isn't as slimy as Verizon is another good idea. Again the majority of Carriers worldwide allow unlocked phones without any restrictions, the most glaring exceptions are Verizon and Sprint in the US.

30. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

And they haven't started to sell LTE only devices yet either. But for the third time, my point is, when they drop CDMA from their towers, there will only be LTE signal, which can support everything a smartphone can do. Once that happens, it will work just like it does on GSM carriers. Is that clear enough for you? And again, that comes down to how phones are registered on networks. On CDMA, they are registered through their IMEI numbers. That is a fact of the technology. Once they drop CDMA, that goes away. Got it?

31. TheOracle1

Posts: 2340; Member since: May 04, 2015

I do get it and you don’t have to be an assh*ole to get your point across. Let me also repeat myself since reading English seems a challenge for you when it comes to anything against Verizon: "... those will require special Verizon software to make voice calls, so that move won't make it any easier to switch carriers with your phone". Do YOU get it? Verizon are planning on retaining their RESTRICTIONS through software.

32. VZWuser76

Posts: 4974; Member since: Mar 04, 2010

If you're so concerned about being an asshole, reread your comments to the author and take your own advice. Did you notice in your quote above that when they start selling LTE only phone, it will need special software FOR NOW? Meaning that it won't be that way forever. And I would imagine the special software is to make it possible to make calls when LTE isn't available, say for instance over WiFi. There are many areas where LTE isn't available but 3G is, so the choice when going to an LTE only device when they're still using 3G CDMA as backup is to either use another option like WiFi calling or they won't have call capability in an area not yet covered by LTE. Again , the info you're quoting says, FOR NOW, meaning it isn't permanent, but a temporary fix until LTE has fully replaced their CDMA network and that gets shut down.

33. TheOracle1

Posts: 2340; Member since: May 04, 2015

Thanks for that Mr.Thought Police. Take your argument up with PCmag: "Even without CDMA, the CDMA philosophy of carrier control of your phone will remain intact." From PCmag Feb. 2015 Or did you conveniently miss that little tidbit?

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