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CMDA vs GMS: Why Your US Cell Phone Probably Doesn’t Work Well in the Rest of the World/Your Non-US Phone Doesn’t Work Well in the US

I got another email from reader Dean looking for cell phone help last night and a bit of research helped me elucidate a great mystery: why US phones tend not to work well outside of the US and non-US phones tend not to work well in the US. I’ve never seen this info laid out plainly so I thought it would warrant a blog post. Please note that this information is very general and personal user experience may vary.

The first thing you need to know is that there are basically two types of cellular networks, CDMA and GMS. Explaining the difference is far beyond the scope of this post. You just need to know that the two don’t play nice with each other.

The second thing you need to know is that GSM is the most prevalent type of network in the world while the US uses CDMA on its two major networks, Verizon and Sprint. AT&T and T-Mobile use GSM. This is why you get poor Verizon service in an AT&T service area and vice-versa.

This is also why my old Verizon phone worked great when I was in the US but was basically useless in Canada, even though I had a plan to use it there. It explains why RV park and motel guests up in Dawson City who were on T-Mobile had no trouble getting service, but the Verizon customers usually had no service. And it also explains why I have trouble getting service in some parts of the US with my phone with both my AT&T and TelCel SIM cards.

Back to Dean’s question. He’s a Verizon customer about to move to Mexico permanently and he wanted to buy a new phone now that would work in Mexico. Mexico’s cellular providers are all on GSM. I was able to ascertain that the phone Dean wants is only available for the Verizon network and is therefore not GSM network friendly. So he needs to instead look for a phone that can run on the AT&T and T-Mobile networks and that is, of course unlocked.

What all this boils down to is:

  1. To have great coverage across most of the US, you need to be with Verizon or Sprint, but your phone likely won’t work well, if at all, in most of the rest of the world (other countries do have CMDA, but, again, GSM is by far the most prevalent);
  2. If you’re coming into the US from anywhere else in the world or are in the US on AT&T or T-Mobile, prepare to have poor service outside of AT&T and T-Mobile network areas.

8 thoughts on “CMDA vs GMS: Why Your US Cell Phone Probably Doesn’t Work Well in the Rest of the World/Your Non-US Phone Doesn’t Work Well in the US”

  1. That was very informative. I live in the US and never understood the cell issue. Thank you

  2. To put it simply, if you carrier is on the GSM system, you need a SIM card. If on the CDMA system, you do not. A new CDMA phone must be programmed with your information before you can use it while with a GSM phone, your information is on the SIM card and is easily transferable from one phone to another. A SIM system with an unlocked phone lets you move to a new carrier simply by buying a SIM card from the new carrier. That is how my unlocked GSM phone works on Rogers in Canada, T-Mobile in the US and on Telcel in Mexico. I just swap cards and buy time as I travel.

    • Thanks for adding that. This is one of the reasons GSM is more popular. This is what I do with my iPhone and why I don’t need a new phone for Europe!

  3. There are phones which have both CDMA and GSM, such as is the case with my very dated Verizon Samsung Note 2 and my husband’s Apple i5. I have used that phone in many countries in Europe and South America with no problem. Just set it to Global mode, insert the local SIM and you are good to go. They were not locked or anything.

    I would definitely check a phone’s frequency band capability online before commiting to anything. Here’s a good site to check compatibility with different phone providers worldwide:
    https://www.frequencycheck.com

    • Thanks!

      I’ll have to see if my iPhone 5C does have CDMA capability. Hmm.

      And frequency? OMG. There’s something ELSE I have to worry about?!

  4. My first cell phone was T-Mobile for exactly that reason–were we about to go traveling in Europe. I also bought a camera about that time and the clerk sold me one that takes two different kinds of memory cards in case either was hard to get as we traveled. That turned out to be a wonderful camera as well.

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