--- title: What Is a SIM and How Do They Work? - Red Pocket Mobile description: What is a SIM card? It's the chip that proves you're a paying customer on a network. Here's what it stores, how it works, and how eSIM changed things. image: https://blog.redpocket.com/hubfs/SIM%20card.jpg --- [![3](https://blog.redpocket.com/hubfs/3.svg "3")](https://www.redpocket.com/) Open main menu Close main menu - [Plans](https://www.redpocket.com/plans) - [Shop](https://www.redpocket.com/shop) - [Activate](https://www.redpocket.com/activate) - [Deals](https://www.redpocket.com/specialoffers) - [Help](https://www.redpocket.com/contact-us) - [Coverage](https://www.redpocket.com/coverage) - [Blog](https://blog.redpocket.com/blog/) [Red Pocket Mobile](https://blog.redpocket.com/blog/tag/red-pocket-mobile) # What Is a SIM and How Do They Work? [Red Pocket Mobile](https://blog.redpocket.com/blog/author/red-pocket-mobile) [Share this blog post on Twitter](https://twitter.com/intent/tweet?text=I+found+this+interesting+blog+post&url=https://blog.redpocket.com/blog/sim-card) [Share this blog post on Facebook](http://www.facebook.com/share.php?u=https://blog.redpocket.com/blog/sim-card) [Share this blog post on LinkedIn](http://www.linkedin.com/shareArticle?mini=true&url=https://blog.redpocket.com/blog/sim-card) ![](https://blog.redpocket.com/hubfs/SIM%20card.jpg) **Key Takeaways** - A SIM card identifies you to your carrier's network, which is what lets your phone make calls and use data. - It stores far less about you than people assume, mainly identification numbers and security keys rather than personal details. - eSIM is now the default on many new phones rather than an emerging option, and the newest iPhone models have no card slot at all. Your [Subscriber Identity Module (SIM) card](https://www.thestreet.com/technology/what-does-sim-card-do-14796633) is a little chip that slots into your phone and lets you access your cellular network. On the front, it features a small gold computer chip encased in plastic, while the back shows the manufacturer's logo. ## **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. What it actually holds is a set of identification numbers and security keys rather than personal information about you. The card presents those credentials to the network, which then checks them against your provider's records to confirm you're a paying subscriber. ## **What Information Is Stored on a SIM Card?** The SIM does play a genuine security role. It stores a secret key used to authenticate you to the network and to help encrypt the connection between your phone and the nearest tower, which is what stops someone nearby from casually intercepting your traffic over the air. It's worth being precise about the limits, though. That protects the link between your device and the network. It is not end-to-end encryption, and it doesn't secure your messages once they've passed into the wider phone system. If you want end-to-end conversation protection, it comes from the messaging app you choose rather than from the SIM. SIM cards are used all over the world. For years, US networks were split between two incompatible systems, GSM and CDMA, and only GSM phones used removable SIM cards. That distinction is gone. The old CDMA networks were retired in 2022, and today every network runs on LTE and 5G, which are SIM-based. Your provider will make sure you have a SIM or eSIM that works on the network you've chosen. ## **How Do SIM Cards Work?** Within each SIM card, two specific encoded numbers make up 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. Network operators use both codes to verify the device's identity, and the success of that verification determines whether 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 to verify 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 if you become delinquent on your phone bill. The ICCID is a code embedded in each SIM card that identifies it as unique. While not a security measure in itself, it can serve to differentiate it from copies or duplicates. 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](https://www.lifewire.com/insert-sim-card-into-smartphone-4777486). This easy method will work for all iPhone models, 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 inserting it into the small hole and pulling it 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 (SIM orientation varies by device). 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. One thing to check first: many recent phones have no SIM tray at all. If you look along the edges and find nothing, your phone almost certainly uses an eSIM instead, and there's nothing to insert. Skip ahead to the eSIM section below. ### **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 a SIM card in place, you can use your fingers or tweezers to slide it out gently. 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, you may need to consult 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 inserted into your phone and your phone is powered on, head to your carrier's website. Today, most carriers offer an [online portal](https://www.redpocket.com/activate) like RedPocket's, where you can use the activation wizard to activate your SIM card in minutes. You'll likely need your phone number or your SIM card number. Thankfully, RedPocket will send you a confirmation code with your SIM card to help make activation a breeze. ## **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 size. Different makes and models take different SIM sizes, and most smartphones of the last decade use nano SIMs. Full-size and micro cards belong to older phones. eSIM, meanwhile, has moved from novelty to the default on many current devices. ### **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 iPhone models, and Samsung Galaxy S22 5G These are examples of which smartphone eras used which SIM sizes, and it's by no means all-inclusive. Worth noting that the newest iPhone models sold in the US don't use any of the physical sizes above, because they have no card slot and rely on eSIM only. If you don't have an Apple or Samsung device and you're unsure which SIM size you need, check your manufacturer's specifications page for your exact model, or ask your provider before ordering. The size of a SIM card does not dictate anything about it other than whether 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. ## **eSIM: The Digital Version** An eSIM does the same job as the card in your tray, except it's built into the phone rather than being something you slot in. Instead of waiting for a card in the mail, an eSIM can be [reprogrammed in the device settings to work with a new provider](https://www.fcc.gov/consumers/guides/esim-cards-faq), usually by scanning a QR code. Setup takes minutes rather than days, which is the main practical difference. There's also a security benefit: an eSIM can't be physically removed from your phone and inserted into someone else's, which closes off one route thieves use to hijack a number. Two things worth knowing if you're shopping. Many recent phones use eSIM by default, and the newest iPhone models sold in the US have no SIM tray at all, so eSIM isn't optional on those models. And if you're moving to a new iPhone, you can often [transfer the SIM from your previous iPhone without contacting your carrier](https://support.apple.com/en-us/118669), a feature called eSIM Quick Transfer. One caution: don't delete an eSIM profile to pause a line. Removing it is permanent, and it can't be restored. Switch the line off in your settings instead. ## **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, they're 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 can hack your SIM card is by calling 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](https://builtin.com/hardware/what-is-a-sim-card). The reason this attack is worth taking seriously is what your number unlocks. Once someone controls it, they receive the login codes that the service texts you, which means they can [access your bank account, take over your email, and lock you out by changing your passwords](https://consumer.ftc.gov/consumer-alerts/2019/10/sim-swap-scams-how-protect-yourself). Every security setting on your phone is beside the point if your number is answering to someone else. ### **How Can I Tell If My Card Has Been Hacked?** If you are unsure whether your SIM card has been hacked, a very SIMple way to check is 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 10 minutes, they can harvest a lot of data and hack into almost any online account they'd like. To improve your security, you should set up a PIN with your carrier or enable two-factor authentication, if available. These would be restrictions you'd put in place to prevent any changes to your account without your consent. The existing security questions are often not secure enough to deter hackers, since, as long as they have your name, most of the information can be obtained through some dedicated internet sleuthing. Two more defenses are worth setting up. Ask your provider about port-out protection or a number lock, which blocks your number from transferring to another carrier until you deliberately turn it off. And limit what you share publicly, since details like your full name, address, and birthday are precisely what someone needs to talk their way through a carrier's identity check. 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 that they can't be physically stolen. ## **The Future of SIM Cards** Modern iPhone models can keep two lines active at the same time, and recent models can run two eSIMs at once, storing several more profiles to switch between. That's genuinely useful for frequent travelers. You can keep your US line running for calls and texts on your usual number while adding a local plan for cheap data wherever you've landed, with no card swapping and nothing to lose in a hotel room. It's equally handy for anyone keeping a work number separate from a personal one on a single phone. ## **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 when 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 let you keep your old phone number on a new account. But you may still be wondering where the best place to get a card would be. At RedPocket, we believe the best place is right here. We have a wide variety of [monthly](https://www.redpocket.com/plans) and [annual wireless plans](https://www.redpocket.com/plans) with unlimited talk and text, plus a range of high-speed data allowances to fit 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. Or skip the mail entirely. If your phone supports eSIM, you can activate digitally and be up and running the same day. ## **Frequently Asked Questions** ### **What is a SIM card?** A SIM, short for Subscriber Identity Module, is a small chip that identifies you to a cellular network. It stores identification numbers and security keys that verify you're a paying subscriber, which allows your phone to make calls, send texts, and use data. ### **What information is actually stored on a SIM card?** Less than most people assume. It stores identifiers such as the IMSI and ICCID, plus a security key, along with some contacts and text messages on older cards. Your billing address, plan type, and usage history live in your provider's systems rather than on the chip. ### **What's the difference between a SIM and an eSIM?** They do the same job. A SIM is a physical card you insert; an eSIM is built into the phone and activated digitally, usually by scanning a QR code. eSIM setup takes minutes instead of waiting for the mail, and an eSIM can't be physically removed from your device. ### **Can I use two SIM cards in one phone?** Yes, on most recent phones. Modern iPhone models keep two lines active at once, typically one physical SIM and one eSIM, or two eSIMs on newer models. It's useful for separating work and personal numbers, or keeping your home line while adding a local plan abroad. Sources: [What Does a SIM Card Do and Why Do You Need One? | TheStreet](https://www.thestreet.com/technology/what-does-sim-card-do-14796633) [What Is a SIM Card and How Does It Work? | Built In](https://builtin.com/hardware/what-is-a-sim-card) [How to Insert a SIM Card Into a Smartphone | LifeWire](https://www.lifewire.com/insert-sim-card-into-smartphone-4777486) [Do I Need a New SIM Card for 5G? All Your Questions Answered | Beebom](https://beebom.com/do-need-new-sim-card-5g/) [eSIM Cards FAQ | Federal Communications Commission](https://www.fcc.gov/consumers/guides/esim-cards-faq) [Set Up eSIM on iPhone | Apple Support](https://support.apple.com/en-us/118669) [SIM Swap Scams: How to Protect Yourself | Consumer Advice](https://consumer.ftc.gov/consumer-alerts/2019/10/sim-swap-scams-how-protect-yourself) ## Related Articles [![](https://blog.redpocket.com/hubfs/eSIM-4-1200x600.jpg)](https://blog.redpocket.com/blog/is-my-iphone-gsm-or-cdma) [Red Pocket Mobile](https://blog.redpocket.com/blog/tag/red-pocket-mobile) ### [How To Tell if Your iPhone Is GSM or CDMA](https://blog.redpocket.com/blog/is-my-iphone-gsm-or-cdma) With the shutdown of GSM and CDMA networks, it is more important than ever to be able to tell if your phone is going to be impacted. By the end of the year, you could... [Red Pocket Mobile](https://blog.redpocket.com/blog/author/red-pocket-mobile) [Read More](https://blog.redpocket.com/blog/is-my-iphone-gsm-or-cdma) [![](https://blog.redpocket.com/hubfs/sim%205g.jpg)](https://blog.redpocket.com/blog/do-i-need-a-new-sim-card-for-5g-red-pocket-mobile) [Red Pocket Mobile](https://blog.redpocket.com/blog/tag/red-pocket-mobile) ### [Do I Need a New Sim Card for 5G?](https://blog.redpocket.com/blog/do-i-need-a-new-sim-card-for-5g-red-pocket-mobile) 5G is almost here! As the major US carriers expand 5G coverage across the US, you might be wondering how you can take advantage of this new, ultra-fast network.  [Red Pocket Mobile](https://blog.redpocket.com/blog/author/red-pocket-mobile) [Read More](https://blog.redpocket.com/blog/do-i-need-a-new-sim-card-for-5g-red-pocket-mobile) #### Make the Switch - [About Us](https://www.redpocket.com/about-us) - [Wireless Plans](https://www.redpocket.com/plans) - [Free International Calling](https://www.redpocket.com/international-calling) #### Get Started - [Activate Your SIM](https://www.redpocket.com/activate) - [Configure Your Device](https://www.redpocket.com/configure-handset) - [Refill Your Account](https://www.redpocket.com/refill) #### Join Us - [Become an Affiliate](https://www.ascendpartner.com/affiliate/registration?refid=293483) - [Become a Dealer](https://www.redpocket.com/dealers) - [Dealer Login](https://my.redpocketmobile.com/) #### Explore - [Government Assistance](https://www.redpocket.com/assist) - [Contact Us](https://www.redpocket.com/contact-us) - [Visit Your Account](https://www.redpocket.com/login) [Follow us on Facebook](https://www.facebook.com/redpocketmobile/) [Follow us on Twitter](https://twitter.com/RedPocketMobile) [Follow us on Instagram](https://www.instagram.com/redpocketmobile/)