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Unlocked Phones and Your Carrier: How They Work Together

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In our last article, we spoke about things you needed to know about buying an unlocked phone, including the freedom of being off contract, and the wide selection of phones available. We also gave you some insight into phone variants and manufacturer warranties. Now, we’re going to discuss phone frequencies and bands, and walk you through some steps to help assure that your new unlocked phone works with your carrier.

A guide to cell network protocols and generations

Cell phone network technology is always improving and getting faster. To help customers differentiate between data speeds, many networks and phone manufacturers tend to group things together by generation. You are probably used to seeing the 2G, 3G, and 4G monikers used. What you may not know is that each generation contains many different small upgrades, and in the case of GSM and CDMA networks, completely different technologies. Below is a chart outlining the family tree.

You may be wondering why there is no 4G CDMA Based Cell Protocol, even though CDMA carriers offer 4G LTE service. LTE is a GSM standard, even though it is used by CDMA carriers like Verizon and Sprint. This is why Verizon and Sprint phones with 4G LTE have SIM card slots.

As you can see, there are different types of 2G, 3G, and even 4G. And, to make matters even more confusing, different carriers and phone manufacturers classify certain protocols as different generations.

For example, in the United States, T-Mobile and AT&T brand High-Speed Packet Access (HSPA+) as 4G. This is why Verizon and AT&T's 4G cell coverage maps look so different in commercials. The reason they do this is that HSPA+'s maximum speed of 42 Mb/s is almost as fast as category one LTE at 50 Mb/s. And the fastest CDMA 3G data protocol, EV-DO Rev. A, tops out at 3.1 Mb/s. So to their marketing teams, it probably made sense to pit 42 Mb/s HSPA+ against 50 Mb/s LTE than the 3.1 Mb/s 3G CDMA network to which customers were accustomed.

However, the rest of the world never had slow CDMA networks to which to compare speeds, and since HSPA+ is really more of an upgrade to Universal Mobile Telecommunications System (UMTS) 3G rather than a totally new protocol, they kept calling it 3G. So, unlocked phones free from AT&T and T-Mobile firmware will not say they are connected to a 4G network when connected to what AT&T and T-Mobile call 4G (as opposed to 4G LTE). You will either see a 3G, H+, or something similar. The reverse is also true if you get an AT&T or T-Mobile-branded phone. Because of the firmware, your phone will register as being on 4G even when connected to what your carrier considers to be 3G. This doesn't affect performance in any way. To make things simple, we use consistent definitions on all our phone write-ups at B&H, regardless of how the phone’s firmware defines certain protocols. Those rules are as follows:

GSM CARRIERS
2G GSM and GPRS
2.5G EDGE
3G UMTS and WCDMA
3.5G HSPA+
4G LTE and LTE ADVANCED
CDMA CARRIERS
2G CDMA and IS-95
3G CDMA2000 and EV-DO

As you will see later, this makes it much easier to determine what phones will work with what network.

Phone frequencies and frequency bands

So, on top of all the cell-phone protocols, there are also cell phone frequency bands. To avoid interference, different parts of the electromagnetic spectrum have been allocated for different uses. Radio stations get one part of the spectrum. Cell phones use higher frequencies than radio stations, and mostly lower frequencies than Wi-Fi, much in the same way radio stations broadcast on only one frequency (93.6 MHz, 99.7 MHz, etc). Cell-phone companies get licenses to use certain frequencies in certain areas. These differ slightly from region to region. To work with your wireless carrier, you phone must support the frequencies your wireless carrier uses. It’s for this reason that all unlocked phones won’t work perfectly with all carriers.

When it comes to 2G, there are only four commonly used GSM frequencies: 850, 900, 1800, and 1900. Fortunately, just about every GSM phone on the market supports all four of those bands. As long as your cell provider has an active 2G GSM network, you don't have to fret about that.

For UMTS 3G, there are five common frequencies used around the world: 850, 900, 1700 (AWS), 1900, and 2100. Here, you need to be a little careful if you are looking for full compatibility, especially if you are on a carrier using 1700 (AWS), as that isn't a widely used frequency band. Still, there are a number of phones that support all five 3G bands, so if you want, you can have it all.

For LTE, things get much more complicated. LTE involves pieces of spectrum that were recently allocated for cell phone use. In the United States we have repurposed much of the old UHF-TV-station spectrum for various cell carriers, through auctions. In the US, certain carriers own rights to certain frequencies that don't line up with the parts of old TV spectrum carriers in other countries use. In fact, there are more than 40 different LTE-frequency bands used around the world—so many, that you can't be sure a phone will work by just looking at the supported MHz. For example, 700 MHz could be Band 17, Band 12, Band 13, Band 28, or Band 44. Each of these uses slightly different frequency allocations, around 700 MHz. For this reason, at B&H we use Band numbers when referring to supported LTE frequencies, as they are far more specific.

For quick reference here are the frequencies used by US carriers:


This table is a screenshot from the B&H e-commerce website. You can also find a full list of worldwide GSM carriers at this link.


How to ensure your unlocked phone will work with your carrier        

Now you know enough to purchase an unlocked smartphone confidently, and all you need to do to make sure your new phone works perfectly is follow these four easy steps.

1. Choose Your Wireless GSM Carrier  If you are in the United States and want to use an unlocked phone, you have the choice of AT&T and T-Mobile primary networks, or any one of the many Mobile Virtual Network Operators that use their networks. Generally, MVNO's have lower prices but have fewer high-end phones available in-store, which is no problem if you bring your own unlocked smartphone. As noted above, Metro PCS (T-Mobile) and Cricket (AT&T) are two MVNO's that are owned and operated by their parent companies as a lower-price alternative.

If you live elsewhere, then pretty much every network uses GSM for voice. Many Canadian networks use CDMA for 2G service, but transitioned to GSM for 3G and 4G, so unless you really want 2G service, you will be fine with an unlocked phone.

2. Check what frequencies your carrier uses  Once you've decided which carrier you will be using, the next step is to check which frequencies that carrier uses. We have compiled a list of most worldwide GSM carriers and what frequencies they use, which can be found here.

So, say you are planning to use AT&T, or any AT&T MVNO as your carrier. Looking at the chart, you can see that AT&T uses 850 and 1900 MHz for 2G and 3G service and bands 2, 4, and 17 for LTE. That means any phone that supports all those frequencies will be 100% compatible with AT&T. It’s quite simple.

Remember that some US carriers call upgraded 3G (HSPA+) “4G”, and LTE “4G LTE”. At B&H, we use 3.5G to be consistent with worldwide standards. It makes it easier, because HSPA+ uses the same frequency bands as 3G UMTS, and is always backwards compatible, where LTE uses a completely different set of frequencies.

3. Choose the right phone variant for your carrier

Now that you know what frequencies your cell carrier uses, you just need to make sure to get a variant of the phone you want that supports those frequencies. The frequencies a phone supports can be found in the specifications tab under cellular network, and will look like this:


This is a sample table of specs from one of the unlocked phones that B&H sells.
 

By looking at this table, you can see this phone supports all AT&T and T-Mobile 2G and 3G frequencies, and all AT&T LTE frequencies, and most T-Mobile LTE frequencies. Furthermore, the LTE frequency it lacks for T-Mobile is Band 12, which is newly deployed and only available in a few markets in addition to Band 4, meaning it will get full coverage everywhere on both AT&T and T-Mobile. As long as frequencies match up, you can be sure that the phone will work fully with your carrier; you just need to insert the SIM card.

4. Setting up your phone  The last step is to make sure everything is set up properly on your new phone. The set-up process will differ from customer to customer, from carrier to carrier. Basically, you need to make sure the APN and MMS settings are set up correctly on your phone, and that your carrier has your account set for the fastest data speeds. With some carriers, these will be downloaded automatically when you first start up your phone. With other carriers, you will need to look up what their current APN and MMS settings are, on the carrier’s website or through sales representatives.

This setup process should be painless, but many US carriers do not train their staff as well as they should when it comes to setting up an unlocked phone. Also, some carriers restrict LTE service based on the International Mobile Station Equipment Identity (IMEI) number listed in your account. Remember, a GSM carrier can't actually tell what IMEI number your phone has unless you tell them, because service is linked to the SIM card. If you are with a carrier that doesn't list your unlocked phone as being LTE capable based on the IMEI number you give them, even though you know it is LTE capable, you can always just give them an IMEI number of a carrier-branded phone. Or set up your account on a carrier-branded phone first, and then switch the SIM card into the unlocked phone.

So, that’s it. Hopefully this has helped. I'm sure you are an expert now but, just in case, if you have any questions feel free to ask the sales staff at B&H, or just comment below.

185 Comments

All this info is making my head spin, but I have had huge frustration trying to figure this out for days, and so has tech support so I'm on my last ditch efforts here to find an answer.

I have a LG stylo 2 unlocked Sprint phone

New carrier is one that uses ATT sim cards.

Data and txt works, voice calls don't work at all.  It says network unavailable.

I really like my phone and just got it paid off with Sprint because my new carrier has not had problems with unlocked Sprint phones. Their phone choices are very limited.... and frankly, I can't afford a new phone.

They gave me the settings for their APN

Please help :(

Hi Jewel!  I'm sorry but I've no idea why your phone is not working properly with AT&T.  For your inquiry, please try reaching out to our experts by e-mailing mobile@bhphoto.com.

Hi,

I bought Samsung Note 8 in Nepal recently. It worked perfectly with local carrier in Nepal. When I tried to use my old US SIM card from verizon, the phone is not working. I called verizon and the agent checked IMEI no and told me the phone is not compatible with verizon. Interestingly, when I put my US verizon sim card, the data works, I can use internet but voice is not working. Appreciate any help.

than You

srawan

I am buying and iphone 6s from someon eI know that said that the phone was from Sprint. But they have used with T Mobile and AT&T and now it's unlocked. I want to use this phone with Metro Pcs, will I be able to?

If the phone worked with our GSM carriers, it should work just fine with Metro PCS too.

Hi!

I got question : I want buy iPhone SE from eBay at&t and said unlocked. Will work phone In my state Slovenia? Couse said must be activated on AT&T prepaid service?? Anyone can explain this?

Ty

This is very interesting. Thank you for posting. How does all this work with the Google Project Fi?

I just bought a new Galaxy S4 that is supposed to be an AT&T unlocked phone. " I had one before and lost it and liked it so bought another"  It came in what appears an original box with factory extras. I had a Steaight Talk plan with the old phone so when I activated the new phone with my old number through Straight Talk the phone works for calls and some text but I cant get data to work. Ive been on the phone with Straight Talk twice so far, first time over 2 hours and second over 45 minutes and data still doesnt work. 

Do you have any suggestions or fixes. 

Hi Steve.

Unfortunately I am not sure what the issue may be.  For your inquiry, please send an e-mail to mobile@bhphoto.com and one of our product agents can assist you further.

Please help!!! Cricket is my carrier and I just need to know what Samsung phone I can purchase to work with my sim card that also has a good camera and internet connection. I really don't  use it for much else. 

Hi Terri!

I've included a link below to a few Samsung phones we carry that will be fully compatible with Cricket and with also the best cameras.

https://bhpho.to/2xectpW

Does anyone have a suggestion for using a Dual sim card ohone which will provide both phone and internet ( waze & email) service in the USA and Israel. We have T mobile in the USA and Golan for israel sim cards.

Samsung is recommending an intenational version Note 5??? Any suggestions, based on experience, would be greatly appreciated.

Moshe

I have been finding the specs of available iPhone 6s's confusing.  They say Vrizon will phase out CDMA in a couple of years, and all the iPhone 6s's seem to have all the bands and modulation types, particularly the SIM-less unlocked version that has all the GSM bands  However, the CDMA carriers and resellers will not activate a non-Verizon phone on their network.  Is there actually a technical reason for this? It looks like they all cover everything (except some don't do Band 30)  I tried a couple of resellers with an IMEI from an A1633 phone, and no dice

Hi,

I am thinking abbout getting an unlocked Samsung Note 8 and my carrier is AT&T.

I currently have a Samsung S6.  Could I switch the sim card OR if I called the AT&T customer serice, and provide them with my IMEI number and no more action needs to be taken?

Thanks so much.

Gracy

You can simply move your Nano SIM from the S6 to the Note 8. On rare occasions you may need to call AT&T and provide them with the new serial number. 

My Verizon galaxy s7 I use with a rogers simple card.  It works mostly but due to the network differences caller ID and msg downloading will not work. I've set all the apn settings, talked with rogers and Verizon but no one can tell me how to fix this problem. Any suggestions?

Hey Greg,

I am sorry but I am not too sure myself.  It's possible that your S7 may just not be fully compatible with Rogers which is why you're experiencing these issues.  

I would like to purchase a Galaxy S8+, unlocked. Am I allowed to switch carriers at any point if I'm not happy with them? I was told they are both AT&T and Verizon compatible. Do I take my phone to the carrier and pay for a SIM card and go from there? Do I need to sign up for a plan? I don't understand any of this. I'm a novice. I'd like unlimited texts and calls. Thank you, Nikki

Hi Nikki

This would really depend on the version of the S8+.  The SM-G955U model can support Verizon and AT&T.  There is the SM-G955F model that does not support Verizon.  With the G955U model, you can have it work with just about any carrier.  However, please note that with big carriers like Verizon and AT&T, you would normally need to have some kind of commitment with them.  Usually a year or two.  

I've been using an unlocked HTC T-mobile G2 for the past couple of years, primarily to use its camera.  I have been unable to figure out how to download apps.  Each time I try, I reach a screen indicating that my Google Chrome browser is no longer supported and then instructing me to use the Google Play Store app to access Google Play.  However, I'm unable to download the Google Play Store app.  My phone is using Android version 2.3.4, and HTC is no longer supporting the G2.  Is there any way for me to successfully update the operating system or to download apps?  Thank you for any help you're able to provide.

This was a really helpful article.  I've just spent 2 hours trying to decide whether to buy a Samsung Galaxy S7 for Tmobile or Unlocked.  I travel out of the country about once a year and would like to not have the T-mobile preloads.  I did all the comparing and the frequencies are all the same for both phones on Samsung's site EXCEPT:  the unlocked has extra frequencies which I assume is fine BUT the T-mobile version has these frequencies that the Unlocked doesn't have:  4G TDD LTE: B38 (2600), B39 (1900), B40 (2300), B41 (2500)

Does this mean I won't have as good connectivity here in the U.S. with the Unlocked version?

Hi Judy - 

That's correct!   You will need to double-check with T-Mobile prior to purchasing an un-locked phone.  But it is best to only purchase a phone that offers the current operating frequencies specified by your carrier.

I have what seems to be a very pecular problem.  I have several Samsung Rugby lll phones(SGH-A997) I purchaced them on ebay several years ago as unlocked phones. I had been using them primarily with AT&T at first, however about 1.5 years ago I also got me a T-Mobile BYOP sim card and activated it to use in one of these phones. Everything was working fine and I was able to switch the sim card from one phone to the other and it would still work fine. No sim card was ever associated with any of the phones IMEI numbers. Now suddenly as of this past Monday, I was no longer able to make or recieve calls on my T-Mobile phone. After two days going back and forth between T-Mobile and AT&T, I find that T-Mobile had recently upgraded their system in this area and it made it where only T-Mobile phones were being recognized on the network. I have been told that the phones I got off ebay have only been partially unlocked and that I am still needing to do a complete unlock to get the phones to work on the T-Mobile network again.  I currently have the requests put in for unlocking these phones and will not know the results for another two days.

Just wondering if you have ever heard of a phone being only partially unlocked, or am I being given the run around for an issue created by T-Mobile, and my only solotion would be to upgrade my phones to T-Mobile phones to continue my service with T-Mobile.

Hi Dave - 

The issue you have described is not unusual. When cellular carriers change or update their systems sometimes 3rd party un-locked devices stop operating correctly.

Dave, I have the same issue.  I was told that I have to purchase a T-Mobile phone to have service in my area.  It took a month to get an answer from T-Mobile as to why my phone was not working anymore.

Hi. I have a prepaid Samsung (Android) phone from AT&T and want to put in a Brazilian sim card and use locally. Brazil = GSM (like AT&T) so it's compatible on that level.  I have a few questions:

1. Does the phone need to be unlocked to work with a Brazilian sim card?

2. How do I know if my phone is unlocked?

3. How do I unlock my phone from abroad?

Hi Kay - 

Type your phone's name followed by "unlocked" into a search engine. Doing so will give you an idea of what most people have experienced in this area. You can also use your phone's model number (e.g., "Samsung Galaxy S6" instead of just "Samsung Galaxy") to narrow the search results.

  • With very few exceptions, Android phones are unlocked by default.
  • Enter your phone's IMEI number into an IMEI-checking service. Some carriers provide a service on their website to determine whether or not your phone is unlocked. You can view your phone's International Mobile Equipment Identity (IMEI) number like so:
  • iPhone - Open Settings, tap General, tap About, and find the "IMEI" section. The fifteen-digit number listed here is your phone's IMEI number.
  • Android - Open Settings, scroll down and tap About device, tap Status, and find the "IMEI" section. The fifteen-digit number listed here is your phone's IMEI number.
  • Most phones - Type *#060# into your phone's Phone app to display your phone's IMEI number. This won't work on Verizon phones.
  • Call your carrier and ask them to confirm your phone's status. If you can't figure out whether or not your phone is unlocked from researching or using an IMEI number-checking service, just call your carrier and provide them with your account details. They'll be able to tell you if your phone is unlocked and, if it isn't, if it qualifies for unlocking.
  • Buy a different carrier's SIM card or borrow one. If you can successfully make a call with another carrier's SIM card in your phone, your phone is unlocked; however, if you can't, the phone is carrier locked and you'll need to talk to your carrier about unlocking it.
  • Before getting a new SIM card, figure out which size of SIM card your phone will use. You can consult the phone's manual, or you can research the phone model using an internet search engine.

Thanks Mark!

Hello,

I purchased an unlocked Samsung galaxy S6 phone and took it to Africa. I'm unable to get 4G because the LTE feature is only compatible with CDMA and not with GSM. What needs to be done to be able to enable and 4G?

This would really depend on the version of your Galaxy S6.  If it doesn't have the radio to support that particular network's LTE band, there is nothing to be done unfortunately.  

I have a Lenovo phab I'm trying to get to work on metro pcs. It worked once then stopped after I tried to use the dual SIM. Now it doesn't work. I've tried the APN fix but then I don't get internet and can't use the 1st sim slot. Any suggestions?

Hi Marieg - 

Please follow Lenovo's instructions for a system reset.

I have an ulocked AT&T phone and am using T-mobile as a carrier. I've have been using it for 2 yrs, no problem. I downloaded the Swiftkey keyboard and my emojiis are black not colored. Swiftkey tells me it probably needs to be updated. I don't get updates because I'm not AT&T. I checked my phone and it says my updates are up to date. What are your thoughts on the subject.

If you have a friend who's on AT&T, maybe try their sim card on your phone to see if you can get any updates from the carrier.  If not and if your phone is up to date, I am not sure what else you can try for Swiftkey to work.

Multi-network phones on Ting(Sprint and T-Mobil) have CDMA frequencies 800 1900.  Similar phones on Verison CDMA have frequencies

frequencies 850 1900.  Will these phones work with both carriers?

Hi Ed - 

Please send us the B&H model numbers to the phones you are considering to the e-mail provided below:

askbh@bandh.com    

Great article - thanks for posting.  I am looking at the Moto 5G plus for use in Europe  (mostly Belgium).  The European build is significantly weaker in terms of RAM/ROM than the US version (2/32 Gb in Europe vs. 4/64 Gb avail in US) so I would like to get the US build. 

My only hesitation is that the US version is only compatible with only 1 of the 2 bands used for LTE in Belgium - 3(1800).  The other band used by the local carrier is 20 (800).  How much should I expect this to impact the performance of the US build Moto 5g plus?

Thanks!

Hi Matthew -

I would not recommend compromising.  Often the additional bands are needed for future upgrades to offered service by the carrier.

Thanks Mark!  Not the response I had hoped for, but certainly appreciated.

Hi,

My cellphone does support LTE, but IMEI says HSPA+. 

I went to my carrier asking to change the plan to a LTE, and according to them I can't because my IMEI, support HSPA+ (which is not true, as I understand is  the sim card which defines if it's HSPA,HSPA+ Or LTE,not only the cellphone) 

They told me that the only option to use LTE service is to change my phone and plan. Which I don't need and I don't want to change. 

My question is, can I obtain a LTE Sim card or buy one? 

I obviously, won't say that to my carrier, because seems like they just want me to buy a new cellphone. 

There's a way to do that? For example, asking my carrier for an replacement. Saying, I lost my Sim card and get a new one, but LTE compatible? 

Thanks!

Hopefully someone understand what I tried to explain.

Hi Martin,

I'm sorry but I don't think we'd be of much help.  We do carry some some cards but they are carrier specific (https://bhpho.to/2sUhFjF).  I'm not sure how to get around your dilemma unfortunately.

This article might be interesting if you have a master in telecommunications but not helpful as a guide to beginners who need help understanding. 

Can I use a Verizon lg g3 on boost mobile, att or t mobile?

 Hi John -

Possibly.  This phone was released about 3 years and has been discontinued for some time.  You may want to contact LG and the carriers directly.

Technology: 4G LTE:

Mobile Frequencies:

GSM 850 / 900 / 1800 / 1900 MHz

UMTS 850 / 900 / 1900 / 2100 MHz

LTE 800 / 1800 / 2600 (Bands 3, 7, 20)

Data transfer:

GPRS, EDGE, HSDPA, LTE, Wi-Fi

I recently tried to purchase an unlocked AT&T S7.  They sent me an unlocked Verizon S7.  My service is with AT&T.  I see that I would lose two bands if I use the Verizon phone.  How big of a deal is this?  I am debating on trying to get it replaced.  Thank you!

Lindsay, I just bought a unlocked GSM S7 phone, but im on the verizon network. Would you want to trade phone for phone? ~ Mike

I have a metro pcs phone I unlocked it if I put a at&t sim card in it from straight talk would it work on at&t tower or metro pcs tower cause where I live cant get signal from tmobile tower 

Billy wrote:

I have a metro pcs phone I unlocked it if I put a at&t sim card in it from straight talk would it work on at&t tower or metro pcs tower cause where I live cant get signal from tmobile tower 

I have the exact same question!  I have an unlocked Metropcs phone that I would like to switch to straight talk to use on at&t tower, do I have to buy an at&t sim card, not the t-mobile sim card? ....did this work for you?

Hi Billy -

It might.  But most likely you will need to activate your phone with AT&T first.

I bought an unlocked iphone 6 and it was unable to connect to At&t network, when I took it to the At&t store they investigated and said it was not compatible, that someone had unlocked it and manually changed it to At&t but that it was not At&t compatible.  Is that possible when we put the sim card in it would work on wi fi just not connect to our network.  From what I'm reading on here it should be compatible if it is unlocked  and 4 G LTE?

Hi MJ - 

Almost anything is possible with a 3rd party un-locked phone.  Your only recourse will be with the selling party you purchased this phone from.

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