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What is a SIM card and what does it do?

By Mint Fox

Originally published July 14, 2022


Last Updated March 12, 2024

Test that reads What Is A SIM Card, below which is an illustration of Mint Fox in a thoughtful pose next to a large SIM card with a question mark on it.

If you’re one of the millions of people who have owned a cell phone in the last several years, you may already have experience with a SIM card. But a lot has changed in the last couple of decades, and in many cases “Does your phone have a SIM card?” is no longer a simple yes or no question.

So let’s go over the basics and start by answering the question, What is a SIM card? If there are any specifics you want to dive into further, we’ll show you where to go for that, too.

In this article

What is a SIM card?

To understand a SIM card, you must first understand a SIM. SIM stands for Subscriber Identity Module. This is the microchip attached to the plastic card that stores your important account information as well as a few other things.

What does a SIM card do?

The SIM card carries the SIM, which connects you to your carrier’s network so you’re able to complete calls, send text messages and connect to mobile internet services. The SIM contains your IMSI, or International Mobile Subscriber Identity, which is an ID number that lets the network know if you have an active mobile account. It also contains a security PIN and your MSISDN, or Mobile Subscriber Integrated Services Digital Network number, which is your phone number. What’s special about a SIM card is that it can be transferred from one phone to another, and this information will follow the card to the new phone. So you can put your card into any unlocked phone and retain your phone number and account services.

This of course means that if a phone doesn’t have a SIM in it, then it doesn’t have a phone number, or an IMSI to connect it to a mobile network. It’s essentially a camera with limited internet capabilities over Wi-Fi only. In addition to these service essentials, your card can store some contacts, text messages, and other bits of key data. If you’d like to know more, we’ve created a pretty detailed breakdown in our blog about what’s on a SIM card.

Text that reads What Are The Different SIM Card Sizes? under which are illustrations of a nano SIM, a micro SIM, and a mini SIM.

What are the different SIM card sizes?

SIM cards used to come in a standard size, which is ironically no longer the standard. As phones got slimmer and manufacturers made efforts to increase memory and battery capacity, they needed cards to take up less space. 

  • A full size (standard) SIM card is 85mm x 53mm
  • A mini SIM is 25mm x 15mm and was the standard for a while after its debut in 1996
  • The micro SIM is 15mm x 12mm and came about in 2003. It became the new standard thanks in large part to Apple introducing it with the iPhone 4
  • The nano SIM is 12.3mm x 8.8mm and made its debut in 2012 with the iPhone 5 

The SIM itself doesn’t actually change in size though. The only thing getting smaller is the amount of plastic surrounding it, to the point where some phones don’t have a plastic card around their SIM at all (more on that later). This means a mini SIM can be cut down into a micro SIM, which in turn can be cut down to a nano SIM (though we don’t recommend doing that on your own). If you fancy yourself a real SIM card history buff, check out our other blog all about the different SIM card sizes.

What’s stored on a SIM card?

SIM cards store data specific to you, like your phone number. They also have an ID number, or international mobile subscriber identity (IMSI), attached to them, as well as a personal identification number (PIN) to protect against theft. The IMSI identifies the phone to your network, letting it know if there’s an active subscription tied to the phone. 

Here is some other information stored on your SIM card:

  • Advice of Charge: This estimates the cost of mobile services for your phone.
  • Authentication Key: This is a unique value for each SIM that authenticates it for the network it’s running on. 
  • Mobile Country Code: This one is pretty aptly-named–it shows which country you’re calling from.
  • Local Area Identity: This is another identifying code combined with your phone number and mobile country code.
  • Service Dialing Number: These are numbers that can be dialed to tell you things like your current balance. 
  • Service Provider Name: This is the name of your mobile operator–like us, for example.
  • Short Message Service Center: This is what delivers your text messages.
  • Unblocking Code: If you’re all thumbs, this is the code needed to open your phone if you keep putting in the wrong PIN.
  • Integrated Circuit Card ID: This is another unique number for your SIM card that helps identify it. Like snowflakes, all of them are unique and no two SIM cards have the same number.
  • Voice-added Services: These are any additional services for your phone that you’ve purchased.

Read more about what is stored on a SIM card.

Does my phone have a SIM card?

Most likely, yes. Years ago, most phones were classified as either CDMA or GSM. CDMA phones didn’t operate with SIM cards, but GSM phones (which at one point had over 85% of the market share) did. We have another blog explaining the difference between CDMA and GSM even further, but all you need to know for now is if you were on a CDMA network back in the day (Verizon is one example), you couldn’t switch your account from phone to phone by just taking out your SIM card, because your phone didn’t have one. But now, as all networks make the switch to be fully LTE (i.e. the 3G network is shutting down and CDMA-only phones will no longer work), most phones released in the last several years operate with a SIM card and feature SIM card slots.

You can refer to your user manual to make sure, or if you have an iPhone you can check out our blog about iPhones and SIM Cards to learn more about your specific model. However, the “tl;dr” version is that all iPhones starting with the iPhone 5 utilize SIM cards. And the newest ones, starting with the iPhone 13, even support eSIM (more on that later).

Smartphone icon featuring a square dotted outline on the righthand side of the screen with an arrow pointing out from it at the outline of a SIM card with the text How Do I Remove My SIM Card? next to it.

How do I remove a SIM card?

Your SIM card is securely locked inside your phone to keep you connected to your wireless network, but it designed to be removable (you know, so you can easily switch to Mint Mobile when you’re ready). Where the SIM card is stored and how you remove it depends on your phone model. 

If you have an iPhone:

  1. Locate the SIM tray. Most newer iPhones have their SIM tray on the right side of the phone, while older iPhones have it on the top of the phone
  2. Insert a paperclip (or a SIM ejector tool) into the hole in the SIM tray and it should pop out of the phone
  3. Carefully remove the SIM tray from the phone and then you can remove the card or insert a new one into the tray

If you have an Android phone:

  1. Locate the SIM tray. Most newer Android phones have their SIM tray along the edge of the phone, on either side or on the top or bottom. Refer to your user manual to confirm the location of your SIM tray. If you have an older Android phone with a removable back piece and battery, the SIM card is most likely in a slot under the battery where you can easily slide it out
  2. Insert a paperclip (or a SIM ejector tool) into the hole in the tray and it should pop out of the phone
  3. Carefully remove the SIM tray from the phone, then remove the SIM card
Text reading How Do I Put in a SIM Card? in the upper lefthand corner, below which in the lower righthand corner is an illustration of a SIM removal tool and a SIM card

How do I put in a SIM card?

When it’s time to insert a new card or replace your existing card, simply locate the SIM tray by following the same steps above. If an old SIM card is in there already, remove it before inserting your new one. Make sure you have the correct size card before placing it in the tray and note the “notch” cut in one corner of the SIM card. This is to ensure the card can only be inserted the correct way. Once it’s been placed in the tray, you should be able to easily reinsert it.

Text reading What Happens When You Switch SIM Cards? under which is an illustration of a micro SIM with an arrow pointing from it to a nano SIM next to it

What happens when you switch SIM cards?

Switching SIM cards is something every cell phone user does at some point, and it’s pretty easy.

There are essentially three different ways to “switch” your SIM card: 

  • Switching a card from one phone to another (moving your phone number and account to another phone)
  • Switching your card out for a new one in the same phone (replacing an old or damaged card on the same account)
  • Switching to a new carrier entirely (getting a Mint Mobile SIM card so you can stop paying so much for that big wireless service of yours) 

No matter which of these switches you complete, what’s important to note is that your apps, photos, music, email settings, and messages all stay on your phone. Pretty much the only thing following your SIM card to a new phone is your phone number and account info. To get really into the weeds about it, you can read our blog dedicated to what happens when you switch your SIM card.

What is eSIM?

While SIM cards are physical plastic cards, an eSIM is a SIM “embedded” in your phone, meaning it’s not on its own removable. Activating an eSIM usually requires scanning a QR code. We dig even deeper into what eSIM is in another blog, including its benefits, security, instant activation (no shipping FTW), and more. Another popular eSIM benefit? Dual SIM capabilities.

What is dual SIM?

Dual SIM means a phone is capable of operating with two different SIM cards (not to be confused with duel SIM, which is when two people stand back-to-back, walk ten paces, then turn and see who can make a phone call first). Dual SIM is achievable thanks to eSIM. Many new devices that support eSIM also have SIM card slots, so you can have one active account on your eSIM and a separate account on your physical SIM card, allowing your phone to make & receive calls from two different phone numbers. And as you may have guessed, we have another blog where we explore the benefits of dual SIM even further.

That pretty much covers everything you could possibly want to know about SIM cards (and maybe even some stuff you definitely didn’t care about). Of course, as we all know, technology is constantly changing and evolving, so by the time you finish reading this blog, there might already be a phone that supports a pico SIM card. You can read all about it in our blog where we explore…just kidding.

Now that you’ve learned all there is to know about SIM cards, maybe it’s time to further expand your phone knowledge. For starters, if you’re ever considering bringing your own phone to Mint Mobile, you’ll want to make sure it’s unlocked so it will work with any SIM card. You can read more about it in our blog covering the difference between locked and unlocked phones.

Or you can take a break from reading for a minute and play a game on your phone. Just make sure it has a network connection in case you get that new high score.

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First published: August 2021, updated July 2022

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