How SIM cards work

We deal with SIM cards on a daily basis in our phones (and sometimes tablets), but yet, we know very little about them. When it comes to SIM cards, all we really know is that when we plug it into the device, it allows us to use our selected carrier’s data network. And when we take it out, we can’t use that cellular network anymore. That’s the basics of how a SIM card works right there, but it’s a bit more in-depth than just that.

Follow along below and we’ll show you all about the SIM card.

What is a SIM card?

A SIM card is a small, tiny little chip (called a card, for lack of a better term), that you can insert into some smartphones. This card holds personal data with your selected carrier, basically allowing the carrier to identify your device and then turn on/off select services based on the plan you have setup with your carrier. If removed from the smartphone, your plan — and data — cannot be accessed (and therefore the phone can only be used, limitedly, on a Wi-Fi network) until it’s put into another phone.

How does it work?

A SIM card contains your personal phone plan data, such as your phone number and your service plan with whatever carrier you have. That’s the neat thing about SIM cards: that they give you some freedom when it comes to your phone. If you end up buying a new phone every now and then, and the SIM slot is the same size as on your original phone, then you’ll be able to simply insert your SIM card and use that as your primary phone, without having to get a new phone number, new data plan, and so on.

Essentially, SIM cards are your personal data plan, in your hands. Since you’re paying for it, it’s a physical piece that you can use however you want to use it. Using a carrier that utilizes SIM cards in their phones will give you the ability to use a different phone for every day of the week if you decide that’s something that you want to do. No need to contact your carrier, because it’s your SIM card, and you’re essentially just moving the home of your data plan around.

Why some companies use SIM cards and others don’t

Before fully understanding what type of phone you’re buying and how SIM cards come into play, you need to know the difference between GSM carriers and CDMA carriers.

GSM stands for “Global System for Mobile Communications,” while CDMA stands for “Code Division Multiple Access.” The difference between the two, in the simplest of terms, is the SIM card (for GSM networks) and the lack of a SIM card (for CDMA networks, although that’s changing). It’s worth noting that while Sprint and Verizon are considered CDMA carriers, they do offer SIMs in a limited sense on a newer LTE phones that will work just fine on any GSM network.

We’ll start by addressing GSM (Global System for Mobile Communications), first. In these phones (or phones provided by AT&T and T-Mobile), you have to keep your SIM card inserted at all times in order to use your data, as your plan is tied to your SIM card. If you want to receive calls, the SIM card must be inserted. If you want to use your available data from your chosen plan while out and about, the SIM card must be inserted. However, you can use the smartphone for apps and internet when connected to a Wi-Fi network.

As aforementioned, the neat thing about GSM phones is that you can change them just about any time you want, without having to contact your carrier, order a brand new phone from them, and so on. If you happen to work in a business where you get to frequently test new gadgets, or you just have access to a variety of different phone styles, it’s nice not having to jump through a bunch of hoops to change things up every once in awhile.

CDMA phones, as mentioned above, do not typically have SIM cards or work with SIM cards (with one exception we’ll point out in a minute). The reason for CDMA phones not requiring SIM cards is because it’s a different technology than GSM, and its radioactivity is also different. To put it simply, instead of your data plan being attached to a portable SIM card for you to use as you wish, your data plan isn’t physically in your hands, but rather your carrier’s, hence the need to contact them if you’re wanting a different phone and are also wanting your plan to carry over.

Although CDMA phones are different for a variety of reasons regarding technology, radioactivity, and all of that, there are some that are being made available with SIM cards, and typically these are for 4G LTE networks. As those networks expand, we’ll be seeing more and more of SIM cards, which overall gives the consumer more freedom with whatever data plan they have.


By following this guide, we hoped we helped you understand just a little bit more about the SIM card inside your smartphone or tablet. It really is a neat technology, and for the most part, makes things so much easier when switching from phone-to-phone or even from carrier-to-carrier (on GSM networks, of course). When you want a new phone, it’s as simple as plugging your old SIM card into that new. Or, when you want to switch carriers, it’s as simple as grabbing a new SIM card from your new carrier.

All in all, SIM cards are a great technology and seem to be the way the world is headed, even as far as CDMA carriers go.

About the Author: Brad Ward

Brad is a tech enthusiast, writing and tinkering with all things technology since 2011. He currently bounces between the LG G3 and his beloved Moto X! His interests include reading, entrepreneurship, the gym, and of course, queso.

  • Richard Dennis

    Hopefully one day phones will be built with both technologies (nexus like) now that carriers don’t subsidise phones there is no benefit to being beholden to a single technology or carrier. And i bet they hate that, but if i got to spend 800 bucks on a phone it won’t be locked into any carrier.

    • Bob DeArmond

      LTE solved your problem a long time ago. All the $800 smartphones today can run on just about any network or carrier around the world.

      • spydie

        Actually they don’t. A lot of flagship phones today are still carrier-dependent. They might be capable of it, I don’t know, but the carriers don’t have them in their list of phones they’ll let you bring to a new carrier. However, I think a lot of $800+ phones lack the complete spectrum of frequencies that’s required to use with all carriers (except iphones, Pixels, Note 3, Moto X PE and a few others)

    • spydie

      Iphones have never been tied to a carrier and run on both GSM and CDMA, as do quite a few other phones like the Galaxy Note 3, Pixel, Moto X pure edition, just to name a few.

      • Richard Dennis

        I did say nexus like. As far as i-phones. Not a chance. Apple ain’t for me. The point i was trying to make is its pointless to manufacture 5 different models of the same device, carriers are no longer subsidizing phones so making carrier specific models is pointless, if i were to buy a pixel i would buy it from Google, not from a carrier, who wants the carrier bloat.

        • spydie

          I agree. Unfortunately, they are still making carrier locked phones with only a certain frequency set in them so you can’t use them on all carriers. I think it’s stupid and I’d never buy another phone that doesn’t have all the frequencies even though I’ll probably never leave Verizon. I have 2 android phones that will work on any carrier and I have two iphones that are the same. My LG G4 will only work on Verizon.

          • Richard Dennis

            You hit the nail on the head for me, i will never buy another phone that doesn’t have carrier diversity. Have a Note 4(still the best Note in my opinion, and still works almost as good as today’s flagships) i don’t like the current pixel line overpriced and non replaceable battery no SD cards. Those are the deal breakers.

            • spydie

              I used to refuse to buy phones without SD card and replaceable battery, but I’ve had to change my tune. It’s getting hard to find a phone with a replaceable battery now (that’s the only reason I keep my LG G4). More phones have gone back to SD cards now, but many still don’t have them. iPhones never did and never will, but I’ve started buying phones with more capacity so I don’t really miss the SD cards now. I thought using the SD card for continuous storage (extension of the built-in memory) was going to be great, but it caused me a lot of headaches and I finally reformatted the card and went back to using it as backup storage. There was no way to use any of the information on the card if it wasn’t in the phone, including no way to transfer the contents to another phone.

  • rvajeff

    Umm… How the heck old is this article? Did you just dust one off from 5 or 6 years ago and copy/paste? Because Verizon has used removable sim cards in all of their phones for at least 4 years now.

    • Justin_Herrick

      We’ve adjusted the post. The ending did state CDMA carriers like Verizon are now embracing removable SIM cards, but the opening was a little off. Thanks for the note!

      • spydie

        You didn’t mention Verizon at all, and you didn’t mention that Verizon phones/sim cards work EXACTLY like a GSM network. Please stop re-posting stories that are many years old. The information in them is pretty useless and out of date.

      • rvajeff

        Sorry but this article still talks about how CDMA carriers typically don’t have sim cards, which is just plain wrong. I’m not sure why you would even recycle such an old post. It’s clear this was written years ago and you just felt like posting it again for some reason. Problem is, it gives people who genuinely don’t know how sim cards work the completely wrong info.

  • Indrajit Ghosh

    Pretty old story, nothing new came up. Sorry wasted my time

  • Daren Naylor

    That is the most technologically inaccurate article I have ever read. A SIM doesn’t carry your plan on it. SIM stands for subscriber identity module – it carries information that your carrier uses to identify which device you are using, and they then allow or disallow services to that device based on your plan. Its called a card, because at first they were credit card sized. And the choice of the word “radioactivity” is plain dumb

    • Abhilash k

      This is the most technologically inaccurate article I’ve ever read.

  • Hh

    What the fuck article

  • Hh

    The author has no idea about GSM CDMA

  • Vipul Sinha

    How do i downvote this post?

  • Raj

    This is written just to write some crap..
    No real information at all..


    Written by just pass …idiot

  • This was meant to be a layman’s introduction into how SIM cards work. Like we said, we adjusted it a bit. Traditionally, CDMA carriers did NOT offer SIM cards until LTE came into play. So, yes, some newer LTE phones from VZ and Sprint have SIM cards that can easily work on any GSM network. Keyword there is “some newer LTE phones.”

    • rvajeff

      “some of the newer LTE phones…” As in, all of them for like 3 years now. Jesus…

  • Darren Falconer

    do you even know what SIM stands for. i would have atleast started with that. and you know some phones arent smart phones and still use SIM cards. let me guess your first phone was an Iphone…