Unlocked Phones and Your Carrier: How They Work Together


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:

3.5G HSPA+
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.


Just trying to figure out what's the minimum percentage of bands that my phone needs to be compatible with:

I've looked up which bands does the Samsung A50 A505GZ work with in the USA and it looks like it's compatible with only 6 out of 9 bands of AT&T. Does that mean that in some areas I won't be able to get reception if my phone will have an AT&T (or AT&T MVNO) SIM card? Or does AT&T work with all 9 bands all the time and this isn't an issue?


Wouldn't a phone that is compatible w/all a carriers network bands be better than a phone that only had one network band compatible? Such as T-Mobile offers three 4G/LTE bands. Would it not be better if a phone was compatible with all three other than a phone that was only compatible w/one of their offered bands? Thank you.

Hi Jan  -  

Theoretically that would be true. I couldn't agree more.  Three is better than one, most of time, in most things.

Is there a phone made that just has all the bands so we don't have to fool around with all this?  

Hi Ja K - 

There really is not one phone that would have all radio bands and be compatible with all of the various flavors and permutations of networked cellular service globally.    For the best flexibility and world-wide compatibility, choose a phone that is GSM+CDMA / 4G LTE compatible.  An excellent choice would be:

The Pixel 3a 64GB Smartphone (B&H # GOPIX3ABLK) from Google (https://bhpho.to/2Jmbmxk) sports premium mobile photography tools without the premium price point.

It has a rear dual-pixel 12.2MP camera and an 8MP front selfie camera which are complimented on the inside by Google's AI camera software features, such as Portrait Mode, which lets you dial in a blurred background for more professional results. As well as Night Sight, which is designed to produce brighter, more detailed low-light images. When capturing video, the rear camera can record at 4K and the front camera at 1080p quality.

The 5.6" 18.5:9 aspect ratio OLED display gives you a plenty of screen for streaming videos, games, or browsing social media while still keeping the Pixel 3a compact and pocketable. The 5.6" screen is housed in a durable polycarbonate frame that you can actually squeeze to activate Google Assistant. With Google Assistant, you can speak your questions and commands to activate phone functions and get answers to your burning questions.

Running the Android 9.0 Pie operating system, this phone is powered by a 2.0/1.7 GHz octa-core Snapdragon 670 processor and 4GB of RAM. You can store your apps and snaps on 64GB of built-in storage. The Pixel 3a also features a Titan M security chip, giving you hardware-based protection for your passwords and OS.

This phone is unlocked and compatible with GSM+CDMA/4G LTE networks. Connect to the internet and other devices with built-in 2.4+5 GHz Wi-Fi 5 (802.11ac), Wi-Fi Direct and Bluetooth 5.0. The Pixel 3a also supports NFC technology for making phone-to-phone data transfers or mobile payments where supported.

Hey, i bought Samsung S6 T-Mobile in USA then i take the Phone with me to Ethiopia but when i insert the sim card i says "Network Lock" What can i do? Please help 

For your inquiry, please contact our tech agent by e-mailing mobile@bhphotovideo.com.  Please also include your B&H Order# in the e-mail.

so I bought an iPhone 6s from a guy online and the carrier says its for metropcs and so I put in my older sisters old SIM card and when I went to go as service to it they said that it was still activated to a different carrier or something was does this mean ?

Hi Joshua -

Bring the phone and the SIM card to MetroPCS to activate the SIM on their service with this newly acquired phone.

Hi! I just bought a new iPhone 6 on eBay that is supposed to be compatible with T-Mobile and its subsidiaries. I am on Metro PCS (now called T-Mobile by Metro or something) and currently have an LG Aristo. Will I be able to just switch my SIM card over to the new iPhone or is it more complicated going from Android to Apple? (I have been a Droid user forever.) Thanks!!

Hi Robin -

The iPhone 6 uses a standard Nano SIM card that should be compatible with your new LG Aristo.

First, thanks SO very much for excellent information that you guys have made pretty clear to the average person. Next, can you guys tell me what Samsung phone ya'll carry that will work with my Metro PCS carrier? Thanks again for so much excellent information1

Kindest Regards,

Lynn Gibson...

Hi, I bought an unlocked Samsung Galaxy S7 Active G891A (provided and then unlocked by AT&T) to use it in Italy.
The phone is really good except for one single thing I have not figured out yet: I can't connect to any LTE service here in Europe. It's not just an indicator thing (I installed a network type indicator from the PlayStore to be sure of it), if I set the Network type LTE the phone goes completely offline confirming this issue.
I have no problem with any other network type (H+, H, edge, gprs...) and I'm sure that the location I live and work are LTE covered by my Italian mvno gsm carrier.
I really don't know how to solve this. Thanks in advance for your patience.

Hi. I have a galaxy s7 unlocked and I'm using cricket. I can't send mms, or use data , I'm unable to update the phone it's still running on 6.0.1 also my phone is reading as its on AT&T. Went to my local cricket store and they said it's my phone that has the issue. Any clue what could be wrong? Thank you

Hey Nikki.  Sorry but I'm not sure.  I would like to suggest to reach out to our tech support team by e-mailing mobile@bhphoto.com.  If you purchase the phone from B&H, please be sure to include the order number.  And if possible, any screenshots illustrating the issues.

Does Switzerland, i.e. swisscom operate on CDMA?

Swisscom is a GSM network.

hello. i have a usa galaxy s8 and i live in canada. i am currently using fido and everything works fine but i want to switch to freedom mobile. i cant use freedom mobile on my device. can someone help me. model number is SM-G950U1. i read something about changing the csc but i didnt understand that

Please help

For your inquiry, please send an e-mail to mobile@bhphoto.com so one of our experts may be able to assist you better.  

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.


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


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.


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?


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.


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.


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


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.


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!


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.

Show older comments