What Is a SIM and How Do They Work?

If you’ve been wondering how your SIM card works (or don’t really understand what a SIM card is in the first place), worry no longer! 

Your Subscriber Identity Module (SIM) card is a little chip that slots into your phone and lets you access your cellular network. On the front, it contains a small gold computer chip surrounded by plastic, while the back shows the manufacturer’s logo.

But what is the purpose of a SIM card, and what’s the science behind this useful little piece of plastic? We’ve got all of the information you need below.

Who Makes SIM Cards?

There are a number of SIM card manufacturers, not just well-known carriers and tech companies. These manufacturers make SIM cards for tons of different phones, and any SIM card that will fit into your phone will work for your phone … for the most part (but more on that later).

What Do SIM Cards Do?

As the name Subscriber Identity Module might suggest, the chip’s primary function is to identify you as a paid customer on your cellular network. The SIM card holds your personal data, like your name, phone number, and wireless plan. The card can then transmit that information to the wireless network you’re trying to connect to prove that you’re a paying customer. 

Phones with no SIM card can’t join a network (and won’t really function like a phone) because the network can not identify the phone as belonging to a specific wireless plan. Without a SIM card, you may as well not have a wireless plan at all. Despite having the right hardware and being in the range of a given network, no SIM card means your phone is essentially a useless piece of plastic. 

TL;DR: When you activate your SIM card, your phone connects to your cellular plan. That’s when the device is linked to your identity and your plan and can start connecting to your network. 

What Information Is Stored on a SIM Card?

Your SIM card ensures that your personal data is stored, though it doesn’t store most of it directly on the card itself. Data like your usage and your account details, such as your address, type of plan, and the number of texts you’ve sent, are not stored directly on the SIM card (with SMS messages as a notable exception). 

Instead, the SIM card gets that data to the provider’s database for more secure data storage. However, the SIM card can access this data when needed. 

You may be worried about the amount of sensitive personal data that the SIM card handles, but the SIM card has a helpful security feature: By identifying specific users, the SIM card ensures that your calls are secure from one end of the network to the other. 

The card stores your device’s unique encryption and decryption key, and this unique code is another way that your phone can be identified by the network. The SIM card encrypts data that is transmitted between your device and the network, including your text messages.

SIM cards are used all over the world. Any phone that needs to connect to a Global System for Mobile (GSM) network needs a SIM card to do so — so every cell phone in Asia, Europe, and Africa and most cell phones in North and South America. The other network option in North America is Code Division Multiple Access (CDMA), which also needs a SIM card. Your provider will ensure you have a SIM card that is compatible with your network of choice. 

How Do SIM Cards Work? 

Let’s get technical. 

Within each SIM card, there are two specific encoded numbers that make it unique to you: the user’s International Mobile Subscriber Identity (IMSI) and the Integrated Circuit Card Identifier (ICCID). The former identifies the user, and the latter identifies the card itself.

The network operators use both of these codes to verify the identity of the device, and the success of that verification determines if the device will be granted access to the network. 

The IMSI is contained in the SIM card’s chip as a 64-bit number. When you initially connect your device to your new phone plan, the SIM card transmits the IMSI and an authentication certificate to your provider in order to identify that the combination of user, device, and plan matches their records.

If this verification is successful, then your device is granted access to the relevant network. However, the IMSI is also how your provider will know which devices to cut service to in the event that you become delinquent on your phone bill payments. 

The ICCID is a code embedded in each SIM card, and it serves as protection against duplication. This 18 to 22-digit code is a security measure for the card itself because it prevents someone from duplicating the card to steal the network identity of the original user.

The number is not a random string of digits; the ICCID is made of several independent numbers, each signifying a piece of information about the card, including the provider network, industry, and manufacture date. The ICCID tells mobile network operators which network to connect with your SIM. 

For example, if your wireless plan is with AT&T in the USA, your ICCID will make sure you’re connected to AT&T’s network, not Verizon’s.

How Do I Insert a SIM card?

Anyone can insert a sim card. This easy method will work for all iPhones, Google Pixels, and newer Androids:

1. Turn your device off.
2. Locate the SIM card tray. This will be on the side of your device, it’s the little rounded rectangle that almost looks like a button with a small hole in it. 
3. Use a SIM card remover tool (or a paper clip or tack) to remove the card tray by gently sticking it into the small hole to pull the tray out. Be gentle: The tray is easily broken!
4. As you pull the tray out, observe the orientation of the hole and the sim card in the tray so that you can reassemble it correctly later. Your tray may have multiple slots (for SD cards), so remember which slot holds the SIM card. 
5. Set your new SIM card into the tray with the golden chip side facing up. 
6. Gently slide the tray back in. Don’t force it, or it may break. 
7. You’re all finished! Turn your phone back on and check that you can send and receive messages. 

What About Older Phones?

If you have an older Android, your SIM card is under the battery inside the phone. The method you’ll need to try is still fairly easy: 

1. Power off the device.
2. Remove the back cover of the device.
3. Remove the battery.
4. Locate the SIM card slot. If there is currently a SIM card there, you can use your fingers or tweezers to gently slide it out. Note this card’s orientation before you do.
5. Insert your new SIM card in the same orientation as the last one: with the chip side down so that it’s touching your device’s internal circuitry. 
6. Replace the battery and back cover.
7. And you’re finished! Power on your phone, and check that you can send and receive messages. 

If your device is from a smaller manufacturer and you can't locate the SIM tray on the side of the device, you may need to reference your user manual or your manufacturer’s website for instructions. 

How Do I Activate a SIM card?

The process of activating a SIM card varies slightly depending on your wireless provider and the make and model of your phone; however, the processes are all extremely similar. Once the new SIM card is physically in your phone and your phone is powered up, head to your carrier’s website.

Today, most carriers offer an online portal just like Red Pocket’s, where all you have to do is utilize the activation wizard to activate your SIM card in minutes. You’ll likely need your phone number or your SIM card number. Thankfully, Red Pocket will send you a confirmation code with your SIM card to help make activation a breeze. 

What’s an Activation Fee?

An activation fee is an added fee that many carriers will tack onto your bill at the beginning of your wireless plan for no reason (other than greed). It covers … nothing.

Largely, activating a SIM card is not something an employee needs to do for you since it’s easy and requires readily available tools. At Red Pocket, we don’t believe in activation fees, so you’ll never find this charge for nothing on your Red Pocket bill. 

Not All SIM Cards Are the Same

SIM cards usually look similar, and at first glance, you may think they’re all the same. However, there are a few key differences you’ll need to watch for when getting a new one. 

The first difference is actually the difference in size. Different makes and models of phones demand different sizes of SIM cards. Most modern-day smartphones will take either nano or micro SIM cards. Full-size SIM cards are more common in older phones, while eSIM cards are so new they’re not quite commonplace yet. 

Which SIM Card Is Best for My Phone?

Here are some examples of devices that can use each SIM card size:

• Full-Size (85 x 53 mm): This predates the smartphone. It’s about the size of a credit card and used to fit into the big, chunky cell phones of the 90s. 
• Mini (Standard) (25 x 15 mm): iPhone 3GS and Samsung Galaxy Ace 
• Micro (15 x 12 mm): iPhone 4 and Samsung Galaxy J series
• Nano (12.3 x 8.8 mm): iPhone 5 or newer and Samsung Galaxy S7, or newer.
• Embedded eSIM: iPhone XR, iPhone SE (2nd gen), later iPhones, and Samsung Galaxy S22 5G

These are just examples of what era of smartphones uses which SIM size, it’s by no means all-inclusive. Every brand and style of cell phone on earth uses one of the above sizes. 

If you don’t have an Apple or Samsung device, and you’re curious about which SIM card size your device needs, a simple Google search will tell you. The size of a SIM card does not dictate anything about it other than if it will fit into a certain kind of phone. All SIM cards have the same capabilities and can hold about the same amount of data. 

The price of a SIM card can also vary from one provider to another. The price of the SIM card is often completely overlooked when purchasing a new phone or a new phone plan, likely because the cost of the SIM card is minor compared to those more expensive items on your bill. 

However, that doesn’t mean you should ignore the cost of a SIM card. The price may be impacted by what type of plan you purchase; if you have limited or unlimited texts and calls.

Do I Need a New Sim Card for 5G? 

You may have heard about major carriers recommending getting a new SIM card for 5G. For example, T-Mobile is offering 5G-specific SIM cards specifically for access to their standalone 5G service. 

Standalone 5G (SA 5G) is a premium 5G service that promises enhanced coverage and uses only the most modern network. For customers already on their 5G compatible plans, T Mobile is offering that new 5G SIM card for free. AT&T and Verizon both offer 5G plans but don’t ask or require you to upgrade your SIM card.

Those two major carriers are not requiring you to upgrade your SIM card for one very good reason: You don’t need a 5G SIM card to use 5G networks if your current SIM card is working on 4G. 

That’s right; your current SIM card is already 5G compatible. When the world transitioned to 3G, SIM cards got an update. From that era onward, SIM cards would be compatible with any generation of network: past, present, and future. Yes, 5G-specific SIM cards are available, but they’re not necessary to access this generation of cellular networks. 

Why Doesn’t 5G Work on My Phone?

Your actual device might not be 5G compatible. For example, iPhones released before the iPhone 12 are not 5G compatible. You might be surprised to learn that some of the first 5G compatible devices were from Motorola and LG in 2018, followed by Samsung with the Galaxy S10 5G in 2019. Apple was actually among the latest to the 5G party. You can check if your device is 5G compatible on your manufacturer's website.

If 5G compatibility is important to you, the first thing you should do is ensure you have a compatible device. 5G’s primary benefit is increased speed. 5G is the next generation of cellular technology, so it’s only a matter of time before 5G is as commonplace as 4G LTE. 

Can a SIM Card Be Hacked? 

The SIM card itself is not a security device. It transmits data and connects to the internet; therefore, it can be hacked. Sim cards are actually a very common target for hackers because, despite containing limited data, it’s linked to your phone number and name. This information will then be linked to your email, social media accounts, and even your banking details. 

Let’s be clear about what qualifies as hacking, though. Hacking is not typically as you see in the movies, with a hooded dark figure and walls of green binary text. 

The most common way someone may hack your SIM card is by making a phone call to your provider and impersonating you. The hacker will convince them to transfer your account to a different SIM card. The hacker would then be able to transfer your personal data to a new SIM card and start intercepting your text messages and calls. This is called SIM Swapping. 

How Can I Tell If My Card Has Been Hacked?

If you are unsure if your SIM card has been hacked, a very SIMple way to check would be to use another person’s phone to call yourself. If the call goes through normally, that’s a good sign. 

You might be thinking, “If my phone weren’t able to receive texts and calls or connect to the cellular network, I’d notice immediately!” And you may be right. However, if the hacker only has access to your SIM card for ten minutes, they can harvest plenty of data and hack into almost any online account they’d like. 

In order to improve your security, you should set up a PIN number with your carrier or two-factor authentication, if available. These would be restrictions you’d put in place to prevent any changes from being made to your account without you. The existing security questions are often not secure enough to actually deter hackers since, as long as they have your name, most of the information could be obtained with some dedicated internet sleuthing. 

Contrary to popular belief, eSIM cards can be hacked too. eSIM cards are just as susceptible to hacking as traditional SIM cards, the only difference is they can’t be physically stolen. 

The Future of SIM Cards

No one can predict the future, but at present, it’s practically impossible to predict a near future that doesn’t continue the use of SIM cards in some capacity. There are no visible alternatives at the moment; nothing else performs the necessary tasks that a SIM card can. eSIM cards are still a kind of SIM card, though they have a slight difference from other SIM cards. 

Still, It’s entirely possible that eSIM cards could become more common than traditional SIM cards. The fact that they can not be removed makes them very appealing to major carriers and manufacturers alike. You can not lose an eSIM and will never need a replacement. 

Plus, that would be one less kind of plastic or digital waste in landfills, which is good for everybody. eSIMs are also even easiest to install, as it’s about as easy as downloading an app: you just scan a code given to you by your provider, then follow the instructions on the screen. No fiddling with paper clips and no delicate tray at risk of damage. 

If anything, SIM cards may even start to play a bigger role in the way we use our cell phones. Apple’s devices are already capable of allowing multiple SIM cards to one device, just not at the same time. 

For example, if someone traveled often, they may seamlessly and independently alternate their US SIM card with their abroad SIM card(s) as they travel without contacting any provider or purchasing new temporary SIM cards when they’re abroad. 

With an eSIM card, it would be possible to use multiple SIM cards simultaneously. In the future, we might see people utilizing multiple providers on the same account with different SIM cards regularly. 

Where Can I Get a SIM Card?

You can purchase a SIM card wherever you can purchase a wireless plan. Your provider will give you a SIM card at the time that you purchase your wireless plan. If you switch plans with the same provider, you likely won’t need a new SIM card. 

However, if you switch providers, then you’ll need a new SIM card. Just because you get a new SIM card does not mean you have to get a new phone number. All carriers now will let you carry your old phone number over to a new account. 

But you may still be wondering where the best place to get a card would be. At Red Pocket, we believe the best place is right here. We have a wide variety of annual and monthly wireless plans that all include unlimited talk, text, and data to fit into any budget. 

Plus, there’s no need to come into a brick-and-mortar store to do it. We’ll mail you your new SIM card along with easy-to-follow installation and activation instructions.

Isn’t it amazing that something so powerful can fit in a flat little envelope and be mailed from Red Pocket right to your mailbox? 

Sources: 

What Does a SIM Card Do and Why Do You Need One? | TheStreet

What Is a SIM Card and How Does It Work? | Built In

How to Insert a SIM Card Into a Smartphone | LifeWire

Do I Need a New SIM Card for 5G? All Your Questions Answered | Beebom