What is a SIM card and how does it work?

When we think about all the things that we can do with our phones, tablets, even some smartwatches – listening to a podcast on the bus, sending an email while in line at the coffee shop, answering a text while walking the dog – none of them would be possible without a SIM card.

A SIM card (which stands for Subscriber Identity Module) is the key that unlocks your connection to cell towers to be able to do, well, anything without Wi-Fi. And if you’re still left wondering what they are, what they do, and how to use them, here’s our simple lesson on SIM cards.  

What is a SIM card

A SIM card is a small, removable chip used in phones, tablets, and even some smartwatches to connect to a cellular network (provided by your carrier.)  

While SIM cards used to also store information like pictures, passwords, contacts, and text messages, they have been greatly simplified since, as most of that information is now stored virtually and can be accessed anytime on any device. Nowadays, they only store important identity and authentication information necessary to connect to a network, as well as your phone number (with some exceptions).  

Note: If you’re using TextNow’s phone service, your phone number won’t be stored on the SIM card, as it’s directly tied to your TextNow account instead, making it even easier to use your number on another device.

How does a SIM card work?

SIM cards are assigned a unique identification number that store information about your phone plan (type of plan, available data, voice minutes, and text messages.) Carriers then use this information to verify your account status and charge you accordingly on your monthly bill.

Whenever you send a text message or make a call – on traditional carriers – the device sends a signal to the network to request access. The SIM card is used to verify that you're allowed to send a text or make a call within that network, and if rejected, the call/text will fail. For data access, the same process follows – if your SIM card is authorized, what you're trying to access will load, if not, you'll get a "no internet connection" error. Simply put, they're key to how a phone plan works.

How does a phone work with a SIM card?

Simply put, when a SIM card is inserted into an eligible device, it allows that device to authenticate with a mobile network (eg. AT&T, TMobile, Verizon, etc.) This simply gives you the ability to connect to a cellular network, but how much you can do on that network still depends solely on your service plan.  

For example, if you only pay for a talk & text only plan, then this connection will allow you to make & receive calls and send & receive texts. But if you were to try to open up a browser or an app that requires an internet connection (like YouTube), you won’t be able to load anything, as your plan doesn’t provide any data connection for them. You will have to rely on Wi-Fi to use any of those other services.  

On the other hand, if you pay for an unlimited data plan, then the SIM connection will allow you to do everything on your phone or tablet, as long as you are within range of the network. Different networks will have different levels of coverage in your area, so it's worthwhile to check what carrier has the best coverage in yours.

Can a phone work without a SIM card?

The simple answer is no. The more nuanced answer would be: Somewhat. Your phone will not be able to connect to a network without a SIM card, therefore, you won't be able to access your number for calls or texts (with some exceptions pertaining to emergency 911 calls and use of SOS mode), or to the internet.

The phone isn't necessarily a brick, though. It can still connect to available Wi-Fi networks, allowing you to browse, stream, even access your phone number in some cases to do everything you need to do on your phone.

What is stored on a SIM card?

Nowadays, SIM cards have limited storage. They mainly store your network plan information (to be able to authenticate and connect you to the network), and your phone number. They no longer store contacts or text messages.

Advantages of a SIM Card  

• Easy device switching – You can easily take out a SIM card and put it into another phone to continue your service without having to contact your carrier. 
• Can activate without internet access – You can activate a SIM card without an internet (Wi-Fi) connection, unlike an eSIM.  
• Can be used in different types of smart devices – While a SIM card is most used in smartphones, it can also be used to connect a compatible tablet, or even smartwatch, to a data network, giving you the ability to continue using those devices outside of Wi-Fi.  

Disadvantages of a SIM Card

Can be lost or damaged – the physical nature of a SIM card means that with frequent removal (or even fraudulent removal), it can be lost or damaged. (Always be gentle when removing and handling it, such as when you reset the SIM card.)  

• Setup can be challenging – While SIM card activations are touted as simple, some can be challenging if you’re not as familiar or comfortable with navigating phone settings.  
• Less convenient for travel – If you have a dual-SIM phone, getting a local SIM card while travelling and activating it is easy. But if you don’t, then the process of taking out your current SIM card, activating a new local one, all while making sure you don’t lose your original one is an added hassle that no one needs while travelling.  

Types of SIM cards  

SIM cards have been greatly simplified since their inception in the early '90s. While there are several types (or sizes) of SIMs available, nowadays, the most common ones you’ll see are the nano SIM and the eSIM:  

• Standard SIM – the largest size chip, now mostly obsolete.  
• Micro SIM – the next generation of chips, smaller in size, but now only seen in older smartphones.  
• Nano SIM – The most used today (also, the smallest).  
• eSIM – A digital SIM built into modern phones (not a physical chip), supported by models like Apple’s iPhone 14 (and up), Samsung Galaxy S, Z, and Note series, Motorolas from 2019 and up, etc.  

How do you activate a SIM card?  

The process to activating a SIM card may vary between carriers, so we would recommend checking out your carrier’s support for relevant instructions. (And if you're getting a new number, check out steps to take before changing your phone number.)

But the general method of activating a SIM card follows these simple steps:  

1. With your phone turned off (or on airplane mode), locate the SIM card tray in your phone and remove it with a SIM ejector tool (a paperclip or back of an earring would also work.)
2. Insert the SIM card into the tray correctly and put it back in the phone.  
3. Power your phone on and wait for it to detect the SIM.  
4. If it prompts for a PIN, enter the one provided by your carrier.  
5. Wait for it to connect to a network (check for signal bars.)  

If you run into any issues with your activation, contact your carrier directly for further support.  

How do you swap SIM cards? 

As long as the phone you bought is carrier-unlocked, you can easily swap SIMs between devices, as your SIM holds your phone number information and carrier information. This means that when you put your SIM into a new phone, you’ll be able to activate it with the same network and make and receive calls and texts without having to do any other external work: 

1. Take out the SIM card from your old device: There is a SIM card tray built into phones that can be pulled out by inserting a SIM ejector tool into a little hole. Alternatively, you can use a paperclip or even the back of an earring to activate the latch through the hole.  
2. Turn your new phone on and put it in airplane mode.  
3. Insert the SIM into the SIM card tray in your new phone.  
4. Turn off airplane mode and wait one minute to authenticate and connect to the network.  
5. If you have any issues, contact your phone provider directly for further instructions. 

Note that as SIMs no longer store contacts or text messages, you will have to restore those using Cloud storage services, like iCloud, or Google Drive.  

What is an eSIM?  

An eSIM is a digital version of a SIM card that's built into your device, eliminating the need for a physical SIM card by scanning a QR code or link provided by your carrier. The “e” stands for “embedded,” which just means that it can be activated on your phone without the physical card. 
Once activated, an eSIM functions just like a traditional SIM card, allowing you to connect to cellular networks, make calls, and use mobile data. 

What does it mean to have a dual-SIM slot?  

Many modern phones will list having a dual-SIM slot, which just means that the phone has two SIM card trays, allowing you to have two active SIM cards in one phone. This is especially convenient if you travel, allowing you to activate a local SIM card without having to make any changes to your current one.  

In some new phones, this dual-SIM slot also includes eSIMs (either for both slots, or one), again, making travelling or even changing carriers a lot more flexible and convenient. When shopping around for the best phone deals, this may be a factor you'd want to consider.

Frequently Asked Questions about SIM Cards: