Technology Explained

What Is a SIM Card and What Does It Do?

Andy Betts Updated 26-04-2019

The struggle with SIM cards can be an annoyance when upgrading to a new cell phone or reverting to a backup. Haven’t we come far enough with technology that such a thing shouldn’t matter anymore? What is a SIM card and what does it do?


Is there a way to use a mobile phone without requiring one? Keep reading to find out.

What Is a SIM Card?

In the world of mobile phones, there are two primary phone types that are available to consumers: GSM (Global System for Mobile) and CDMA (Code Division Multiple Access). GSM phones use SIM cards while CDMA phones do not.

SIM cards are the small cards which contain a chip that you need to insert into a GSM phone before it will work. Without a SIM card, a GSM phone won’t be able to tap into any mobile network. The card holds all the critical information.

what is a sim card
Image Credit: Emilie Gerard/Shutterstock

By comparison, CDMA carriers keep a list of all phones that can use their network. Phones are tracked by their ESN (electronic serial number) so they don’t need SIM cards. Once activated, a CDMA phone is tied directly to that particular carrier’s network.


In the United States, most mobile carriers provide CDMA phones. The two main exceptions are AT&T and T-Mobile, which both provide GSM phones. Internationally, GSM is the more popular technology by a landslide, due to legislature and industry influence that nudged providers towards using GSM.

What Does a SIM Card Do?

what does a sim card do
Image Credit: Rrraum/Shutterstock

What information does a SIM card hold? The most important bits of data include the IMSI (International Mobile Subscriber Identity) and the authentication key that validates the IMSI. The carrier provides this key.

If you’re interested in the nitty-gritty, SIM authentication goes like this:

  • On startup, the phone obtains the IMSI from the SIM card and relays it to the network. Think of this as the “request for access.”
  • The network takes the IMSI and looks in its internal database for that IMSI’s known authentication key.
  • The network generates a random number, A, and signs it with the authentication key to create a new number, B. This is the response it would expect if the SIM card is legitimate.
  • The phone receives A from the network and forwards it to the SIM card, which signs it with its own authentication key to create a new number, C. This number is relayed back to the network.
  • If the network’s number A matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted.

Long story short: this data not only determines which network to connect to but also acts as the “login credentials” which allow a phone to use said network.

Switch Phones With a SIM Card

For this reason, SIM cards are actually quite convenient when it comes to switching phones. Since your subscriber data is on the card itself, you can plug the SIM into a different phone and all will be well. On the other hand, switching phones with a CDMA carrier is more difficult since the phone itself is the entity that’s registered with the network.

switch phone with sim card

Each SIM card has a unique identifier called the ICCID (Integrated Circuit Card Identifier), which is stored in the card and engraved upon it. The ICCID contains three numbers. There’s an identifying number for the SIM card issuer, an identifying number for the individual account, and a parity digit that’s calculated from the other two numbers for extra security.


SIM cards are also capable of storing other information, such as contact list data and SMS messages. Most SIM cards have a capacity of between 32KB to 128KB. Transferring this data mainly involves removing the SIM card from one phone and inserting it into another, though this has become less important with the advent of backup apps.

However, SIM card storage is now dwarfed by internal phone storage capabilities, so SIM cards really have no use other than to grant access to specific networks now.

What Is a Locked SIM?

what is a locked sim
Image Credit: Mikhail Mishchenko/Shutterstock

When your phone is locked to a particular network it’s actually the phone itself that’s locked. Not the SIM card.


In practice, GSM carriers can implement software on phones such that a particular phone will only accept a designated SIM card from a particular network. If the phone and SIM card do not match up, the phone won’t operate. This is what it means when a phone is “locked.”

Unlocking a phone, then, removes this limitation so a phone can accept SIM cards from other networks. If you ever plan on selling your phone, this is an important consideration since you’ll need to unlock it before the buyer can use it. The same applies if you’ve bought a locked phone.

Pre-Paid SIMs

There’s also one other important point to consider: pre-paid SIM cards. These pay-as-you-go SIM cards do not require a subscription or a contract. They also tend to be cheaper, especially if you purchase them from an MVNO. These can be immensely useful if you travel internationally and want to avoid expensive roaming charges.

How much is a SIM card like this? At most it’s a nominal fee. But you’ll often get it for free when you pay for your first chunk of airtime allowance.

A guide to unlocking is outside the scope of this article, but you may be interested in buying unlocked phones Never Buy a Phone From Your Carrier! Buy Unlocked Phones and Save Hundreds Unlocked phones can cost a fortune, but the savings more than make up for it. You can save thousands by not signing a contract. We've got six phones that come contract free! Read More which can save you hundreds of dollars. We’ve also shown how to fix the “SIM not provisioned MM2” error How to Fix the "SIM Not Provisioned MM 2" Error Problems getting your new SIM card to work? The "SIM not provisioned MM2" error message is frustrating, but simple to fix. Read More .

The Rise of eSIM

At some point, the industry is likely to move to the eSIM. This is where the SIM is embedded in the phone and activated remotely by the carrier, meaning you no longer need to fiddle with the tiny cards. It’s already in use for the second SIM in some dual-SIM devices, like the iPhone XS. Although you still need a card for your main SIM.

But it will take some time before this becomes standard. Until then, take a look at some apps that can help you manage your SIM card data for additional help. Also, learn about ways in which your SIM card can get hacked to protect yourself from such scenarios. And if you need a new phone plan, check out the best unlimited phone plans.

If you’re curious about eSIMs, read our introduction to eSIM and how it works What Is an eSIM and How Is It Better Than a Standard SIM Card? Buying a new phone? Your SIM card will need upgrading to the smaller eSIM card. What is an eSIM and why is it being introduced? Read More .

Affiliate Disclosure: By buying the products we recommend, you help keep the site alive. Read more.

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  1. Robert D'Andrea
    October 31, 2019 at 5:51 pm

    I did enjoy reading this article. Everything you ever wanted to know about the SIM is there.

  2. Nerf
    September 26, 2017 at 2:30 am

    Sadly my question wasn't answered,("How can you get into your phone that needs a PUK code, but you never set one up before, what would the code be for and alcatel one touch?") if someone could answer my question I would be very appreciative! :)

  3. miss june
    December 27, 2016 at 12:38 am

    I deactivated old phone which I got in may 2016 because I had 2 cells. WHAT SHOULD I DO WITH IT TO BE SAFE
    I was told give to police to use for domestic violence siyuations, would this be safe or should i smash it ,


  4. kathy
    August 4, 2016 at 10:25 pm

    why don't canadian carriers provide pre-paid sim cards to dual-sim phones? I am told none of them will.

  5. jada
    December 9, 2015 at 4:09 pm

    if i was to make a new type of program to put in TV's, would i be able to use tech like this to do it or is it trademarked?

  6. Jaquie
    December 7, 2015 at 11:34 pm

    I have a sim card from my old iphone through Verizon. Can I put it in a refurbished Galaxy S4 that comes with no sim card, that I would also put on my Verizon account? Or does it even need one?

    • Seth Medl
      February 21, 2016 at 5:21 am

      Well it depends. Does the old SIM card work (like can you reactivate/let it connect to a network and have its own number)? Is the Galaxy S4 from Version? If the SIM card works and the Galaxy S4 is from Version, and has a SIM card port (I'm not to sure), then I don't see why it wouldn't. If any of the two questions is a no, the no it won't work

  7. Yoko Matthews
    May 25, 2015 at 4:20 pm

    I'm very mach appreciate information you provided . Thank you.+

  8. Anonymous
    February 23, 2015 at 3:25 am

    If I buy a prepaid phone here in Canada, can I send it to the Philippines? And are they able to use it if they change the SIM card?

  9. mark frazier
    February 2, 2015 at 10:13 pm

    Thanks for the explanations I currently have a sprint replacement off eBay and upon activation was informed by tech activating phone that a new sim for Samsung mega6.3 sprint specific phone was needed too have LTE service obviously not true as phone is getting and stays connected too LTE service they seem just unable too communicate too reused sim card even with the part number and identification number of what's in the phone now. PS I didn't tell the tech until after they rendered the number and service over. Other than the phone sending a message too contact carrier or bring too sprint store works as if phone was reading it I am assuming it still contains my info sprint just can't access it? Any insight on this anyone

  10. janet
    January 19, 2015 at 7:08 am

    I had a cell phone once and the screen went funny, so I switched it off and back on again and it was fine, hope this helps other users

  11. Alejandro
    January 14, 2015 at 12:42 pm

    Firstly, Great article!

    I have a doubt... What happens to the IMSI when you change carriers but maintain same phone number?

    Ex: I have a contract with Carrier A (phone with SIM card) and I decide to switch to Carrier B (phone with SIM card too) but I maintain my phone number. Would the IMSI change since I switched companies or would it be the same? I kept the same phone number but I had to change my SIM card...

    • Caroline
      January 20, 2017 at 10:12 am

      Your IMSI will change since this is a code linking you to an operator, or how to make yourself known to an operator. If you'd try to log in for instance with your current SIM to another network (trying to perform a Location Update on another network), then you would not be able to do this since the IMSI is not recognized. On signalling level, one of the ways your SIM talks to the network, your SIM will receive 'IMSI not in HLR/VLR'.

  12. ttocs
    December 18, 2013 at 10:11 pm

    the trouble with porting a number on the CDMA phones is that it takes a REALLY long time to do took 9 days when i recently did this for my husbands new phone . during that time neither phone was usable. it was a pre paid plan so maybe its quicker if you are using a specific carrier.

  13. Dan M
    December 9, 2013 at 4:32 pm

    Hmm.. Having issues with separating paragraphs..

  14. Dan M
    December 9, 2013 at 4:31 pm

    I think there is a little misinformation going around:

    Verizon 4g phones use SIM cards, they are also Global phones so they will work overseas.

    CDMA carriers (at least Verizon) cannot deny you service for a phone they did not sell you. This is part of their agreement when they bought analog spectrum from the FCC.

    Getting a new CDMA phone will not cost you your number. Number porting means that a user is not locked into a carrier and if they have a new number they most likely either chose to have a new number or did not complete their contract with one of the carries.

    Unlocking phones is still a touchy (and illegal) process in the US. Carriers can and should unlock your phone when you complete your contract, and according to the new head of the FCC, he would like all carriers to offer some sort of unlocking for paid off contracts. He has threatened to legislate if the carriers do not come up with a solution on their own.

  15. rigurat
    December 9, 2013 at 11:00 am

    That's why I got T-mobile instead of Verizon because they use SIM cards. I just buy the prepaid SIM, pop it in my international phone and enjoy. When I travel abroad, I just buy another SIM for that country.

  16. Jackson
    December 8, 2013 at 8:25 pm

    Looks like the article was a hit! Awesome work, Joel!

  17. Brandon R
    December 8, 2013 at 3:50 am

    Interesting article however I want to learn how to unlock phones it will be great if this website provided on article or manual on how to unlock phones.

  18. Ethan
    December 7, 2013 at 3:13 am

    Very informative article.

    In your explanation of the phone connecting to the network, I believe it should say that B and C need to match, not A and C. So it should read:
    "If the network’s number B matches the SIM card’s number C, then the SIM card is declared legitimate and access is granted."

    If A is the random number, + is the signing operation (for example), and K is the key then you'd get
    A + K = C on the phone and
    A + K = B on the network.
    A is sent between the two and the network wants to verify that the phone has the correct key, K. It does that by comparing B to C and if they are the same then the network knows the phone has the correct key.

    • Joel L
      December 13, 2013 at 10:33 pm

      Whoops, you are correct. Thanks! :)

  19. android underground
    December 6, 2013 at 8:02 pm

    SIMs are great, but it's time to ditch the plastic. Why can't I just download a SIM 2.0 from my carrier and start calling? It would make traveling much easier, 'cos with a bunch of virtual SIMs stored on my phone I could just change to a local carrier with a tap on the screen. I could even use an app to automate the process so my phone would use my location to pull the cheapest SIM from my collection.

    CDMA? Hell no! If the carrier doesn't like your phone they can keep you out of the network and make you buy one of their phones. And forget about switching to a local carrier to avoid the overpriced roaming rates. CDMA puts too much control in the greedy hands of the carriers.

    • John
      December 12, 2013 at 12:16 am

      Biggest issue would be security. SIM cards are encrypted and the technology is difficult to hack. If it's just an app secured with a username/password, someone could hack your phone and make/receive calls as you.

  20. Simon
    December 6, 2013 at 6:56 pm

    Thanks for this article! First I thought I won't learn anything new about SIM cards, but then I read that most mobile carriers in the US provide CDMA phones. So now I know why you can't use whatever phone you want with a specific carrier there. I'm from Switzerland and we only have SIM card phones as far as I know. Which I personally think is better, because if you have an unlocked device (you can get one if you don't buy it at your carrier's shop, but at an independant reseller), you can use any SIM card you want (that's useful for selling and switching the phone and using it with a prepaid card when on holidays).

  21. Zhong J
    December 6, 2013 at 6:51 pm

    There's also a regular sized SIM card and a micro or nano size SIM card, could you explain the difference between the two? SIM cards also have the ability to save contacts and other phone information in it, which doesn't make it useless.

  22. Dex
    December 6, 2013 at 6:16 pm

    It's not that hard to restore your SIM card if you lose it. It takes about 15 minutes at the your carrier's office. Most of the time all they need to know is 3 last dialed numbers and your phone number. They don't even charge you for replacing a sim card. Personally, I prefer SIM phones to CDMA phones as I can keep a number for as long as I like. I find it odd when people on facebook are like "Got a new phone with a new number, text me" etc. I've had my phone number for the last 15 years and changed about 8 phones.

    • Joel L
      December 13, 2013 at 10:29 pm

      I'm also puzzled by most "got a new phone, need numbers" posts, especially from people who have SIM cards but don't use it to store contacts.

  23. Kevin M
    December 6, 2013 at 5:55 pm

    This technology is so out of date! These cards should be obsolete and instead this information should be stored on a secure server system with redundancy. The idea that an inherently unstable storage device is used for all my important information is a joke! Phone companies that are to cheap to upgrade these systems does not deserve our business!

    • furrynomous
      December 9, 2013 at 4:19 am

      erm...? Then how do you connect to your network? Would you keep a server with you all the time (LOL)?

    • Joel L
      December 13, 2013 at 10:28 pm

      Without a SIM card, how will the network know if you are a legitimate user? It's either the SIM card for authentication (GSM) or the phone itself (CDMA). I doubt a username/password system would work well for a cellular network, unfortunately.

    • Kevin M
      December 13, 2013 at 11:09 pm

      All this is easier than you think, furrynomous ever heard of public gateway? Even if you do not have a phone number, if you have a cell phone it can still connect to the towers within the public gateway with a key that is coded to the phone. Joel L - it is with this method that that handshake could identify your phone and would use IPv6 DNS which is already in widespread use and every device can have their own IP. That IP is then registered through your cell provider and your access to the network is granted through that IP and would not need the SIM card. Its the same way that a website is found and how your browser is then able to connect with it.

      I am not saying that you need a user name and password because you would not and using that would add an insecure layer if the password was transported without security. The authentication would instead be routed by way of your IP which identify's the phone. How each network decides to implement it would be on them and if they want to use a user name password system they could and as long as they put the proper SSL in place it would work.

  24. Bob
    December 6, 2013 at 5:30 pm

    I have two AT&T iPads which have SIMs. Can I swap SIMs in my iPad and connect to the network same as a phone does?

  25. Claire C
    December 6, 2013 at 4:52 pm

    I changed to a CDMA network from a GSM network, and the one thing I really regret is the lack of a SIM card. I used to move my SIM card from phone to phone, depending which phone I wanted to carry (and which was fully charged, etc.) It was very convenient.

    Now, if I'm about to go out, and notice that my cell phone charge is low, I have to either not carry my phone, or run the risk the battery will become too low to function. Very annoying. I used to just move the sim to a different phone and head out.

    Also, sim cards (and an unlocked phone) allow you to just slip in a different sim if you are traveling out of the country. No need for roaming agreements or anything like that.

    • Joel L
      December 13, 2013 at 10:26 pm

      That's exactly why I love having a SIM card. Too bad your new network doesn't support it. :(

  26. Matthew
    December 6, 2013 at 4:38 pm

    One other variation on the Locking issue, and yes, unlocking is a HUGE topic, as while many networks will unlock for a fee (most UK networks it's about £15), some phones are easy to unlock for free.

    The variation though, there are 4 UK network carriers, Vodaphone, O2, Three and EE (EE used to be Orange & T-Mobile).
    Generally, a virtual operator SIM will work in phones locked to it's man carrier network, but the reverse is not possible.
    So, a Virginmobile (UK) SIM should work in a T-Mobile phone - but not an Orange phone, despite the merger. apparently newer EE phones do support T-mob and orange SIMS (and maybe virtual operators on either flavour), though other reports suggest that EE = T-Mobile + Orange roaming.

    It is far better to have an unlocked phone, as it means the opportunity to use a local PAYG SIM when abroad for cheaper calls, or to disloyally jump when another network offers a better deal.

    In most services, loyalty is NOT rewarded, good PAYG tariffs turn bad, and if reaching the end of a contract, it pays to be "thinking of leaving" - very rarely would any operator just say goodbye, so you can expect some incentive to stay, such as matching a genuine offer from elsewhere, maybe with a bonus thrown in

    • Joel L
      December 13, 2013 at 10:25 pm

      I think you're right. Having an unlocked phone is much better than a locked phone, especially since customer loyalty is not a huge aspect of Western culture (like you said). Options are always nice to have.

  27. James
    December 6, 2013 at 3:59 pm

    Verizon network doesn't use sim cards.

    • Rich
      December 6, 2013 at 5:59 pm

      Uh, I have 3 phones from Verizon and ALL 3 use SIM cards.

    • Howard B
      December 6, 2013 at 6:01 pm

      Verizon network also uses CDMA, not GSM; GSM phones have SIM slots. Some few Verizon phones DO have SIM slots, however.

    • Gyps
      December 6, 2013 at 7:21 pm

      Verizon does use SIM Cards..

  28. Joseph de Souza
    December 6, 2013 at 2:27 pm

    In India, and probably some other countries too, SIM cards are used in CDMA phones as well

    • sadiq
      December 6, 2013 at 3:07 pm

      On CDMA its called UIM

    • Shubham Shukla
      December 6, 2013 at 4:09 pm

      We don't use sim cards in CDMA phones here in India. Who told you ? O.o

    • Shreya Dahal
      December 6, 2013 at 5:45 pm

      In Nepal, CDMA phones are usually provided by the Carrier, so I guess it does not have a sim card. But I have seen many CDMA phones that do have one. Infact there is one carrier here that only provides a CDMA sim card that can only be used for internet access, you can put it in a phone or get a module for the computer.

    • CJ
      December 7, 2013 at 1:56 am

      Yes Joseph I was just about to mention it. In India we have CDMA phones with SIM Cards...
      I have my own Reliance CDMA which has SIM card and so I was able to change my phone number once. [Obviously by changing the SIM card]...

      @Shubham, u might not have noticed it buddy.....

      This is also because we have universal brands of CDMA phone, one of which is Samsung... So, we need a SIM card that can be used with these phones.... ;-)

  29. sam
    December 6, 2013 at 2:04 pm

    Interesting article, great on the tech details, but the entire article is ABOUT SIM cards, yet nowhere do you spell out what the acronym means. That might be a little helpful.

    SIM - Subscriber Identification Module

    Just thought defining the basics of what you're discussing here might be helpful.

    • Joel L
      December 13, 2013 at 10:24 pm

      Haha, yikes. I was certain that I included it in the article but it looks like I did not. Thanks for catching it!