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What is a SIM card and how does it work?

Read all the information about what a SIM card is, learn about its advantages, how it works, types of SIM, and more!


July 26, 2023

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SIM card is a tiny card in your phone that holds the key to your entire mobile identity. SIM cards have been the essential parts of smartphones for decades. They enable us to connect to cellular networks, make calls, send SMS, and use mobile data. However, things are changing and some say it’s time to retire this technology in its current form. 

In this article, we’ll explore what SIMs are, how they work, what are the different types available, and what the future holds for them, as SIM transforms into eSIM technology.

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What is a SIM card: Definition

A SIM card, or Subscriber Identity Module, is a small, removable chip used in mobile devices to store data your mobile device needs to connect to a mobile network. Notice it says mobile devices and not just mobile phones? That’s because, while it is mostly used in mobile phones, it can also be used in tablets, smartwatches, mobile hotspots, some laptops, and IoT devices. However, let’s focus on cell phones and their SIM cards. 

What data do SIM cards have that allows them to connect to a mobile network? Here’s what it contains:

  • International Mobile Subscriber Identity (IMSI): This is like your phone’s personal ID, which helps the network recognize your device.
  • Authentication Key (Ki): This key authenticates your phone to the network so it can have secure access to it.
  • Phone Number (MSISDN): This is your actual phone number. It allows you to make and receive calls and texts.
  • Carrier Information: Settings and configurations specific to your service provider.
  • Contact Information: It can store a limited number of contacts, although most people now store contacts directly on their phones or in the cloud.
  • SMS Storage: Some SIM cards can store text messages, though most are now stored on the phone.

How do SIM cards work?

From the user’s perspective, this is simply a matter of popping your sim card into your phone, and it starts working automatically. Most phones have a small tray on the side or back that pops out when you use a special pin or paperclip. You just need to place the SIM card into the tray and slide it back into the phone. 

Very likely, your carrier will send you the instructions to activate your SIM as soon as it is detected on the network. This is an automated process, but sometimes you might need to call a number, visit a website, or send an SMS. Sometimes, resetting your phone is necessary.

If you’re switching from an old SIM card, transfer your contacts and SMS messages if they are stored on the old SIM.

This is how you use your SIM card and make it work. However, what is the process that happens in that tiny chip?

  1. Identification. Once you put your SIM into your phone, it sends its IMSI to the network, identifying you and your subscription.
  2. Authentication. Then the network looks at the key (Ki) stored on your subscriber identity module (the long name for your SIM), so it can give you secure access.
  3. Network Access. Then this exchange happens successfully, your SIM connects to the network and you’re good to go.

Types of SIM cards: formats and sizes

Phones got smaller and leaner, and SIM cards had to follow. That is why their design and size changed a lot over time.

Mini SIM Cards: The Mini SIM card, also known as the standard SIM, measures 25mm x 15mm and was introduced in 1996. It is not used very often anymore and has been replaced by smaller SIM card formats.

Micro SIM Cards: Introduced in 2003, the Micro SIM card measures 15mm x 12mm and is more compact than the Mini SIM, making it popular in smartphones from around 2010 to 2015.

Nano SIM Card: The Nano SIM card, introduced in 2012, is the smallest of the three, measuring 12.3mm x 8.8mm. It has very little plastic surrounding the chip, which helps when designing a thin and lean mobile device

eSIM: The eSIM, or embedded SIM, is a digital SIM technology introduced around 2016 that eliminates the need for a physical card. Instead, the SIM data is stored directly on a chip embedded in the device, allowing users to download and switch mobile plans from different network operators easily.

Different types of SIM cards
Types of SIM cards. Source: A1 Bulgaria

The following table lists SIM card types, which are easy to tell apart by looking at their format and size.

Type of SIM cardSize
Traditional SIM85.6mm x 54mm
Mini SIM card25mm x 15mm
Micro SIM cards15mm x 12mm
Nano SIM card12.3mm x 8.8mm
Embedded SIM (eSIM)6mm x 5mm
Table 1: Various SIM card sizes.

The SIM card changes have undoubtedly brought multiple benefits for manufacturers of iPhone and Android devices. The most important is the possibility of improving the user experience with increasingly compact and functional designs.

Which SIM card can I use on my Android or iPhone?

Different iPhone and Android models are compatible with different SIM card formats. Here are some of the most popular devices and their compatible physical SIM cards.

iPhone and Android modelsType of SIM card
iPhone 4 and previous modelsMini SIM cards
iPhone 4s to iPhone 6 PlusMicro SIM cards
iPhone 6s to iPhone XNano SIM cards
iPhone XR, XS, XS Max, iPhone 11, 11 Pro, 11 Pro MaxDual SIM, Nano SIM, and eSIM card
iPhone SE (2020) and more recent modelsDual SIM, Nano SIM, and eSIM cards
Android devices prior to 2011Mini SIM
Android devices from 2011 to 2012Nano SIM card
More recent Android devicesNano SIM / eSIM card
Table 2: SIM card compatibility according to the phone model

Compatibility also has to do with the geographic region and the operator. We recommend you verify this information before making any purchase decision, especially in other countries.

For example, the iPhone 14 in the United States already comes without a slot for a removable SIM Card. In other countries, this same model is dual-sim and accepts conventional SIM Cards as eSIM cards. It would be more practical to have this device and not the one you bought abroad.

Types of SIM cards: GSM vs CDMA

There are two types of SIM cards based on their technology and standards used by mobile networks. GSM (Global System for Mobile Communications) and CDMA (Code Division Multiple Access). 

  • GSM cards are the most common type. They’re super convenient because you can easily swap your SIM card between different phones, keeping your phone number and services intact. Plus, GSM networks support international roaming, making them great for travelers.
  • CDMA cards are different. Traditionally, CDMA phones didn’t use SIM cards, as your phone number was programmed directly into the device. But nowadays, with the rise of LTE technology, CDMA carriers also use SIM cards for 4G and 5G services. These SIM cards work similarly to GSM but are usually tied to a specific carrier.

So, GSM cards are great for flexibility and traveling, while CDMA cards are known for their solid network performance, especially in certain areas.

Types of SIM cards: Virtual SIM vs Physical SIM cards

Virtual SIM cards have many names, including digital SIM cards and embedded SIMs, but the most commonly used name is eSIM

So, what is the difference between an eSIM and a SIM?

Unlike traditional physical SIM cards, eSIMs store all their data in a chip embedded in your device. This means you can download and switch mobile plans directly on your phone without fiddling with tiny SIM cards. 

The biggest perk of eSIMs is convenience. Setting up an eSIM is super simple, and you can switch carriers quickly without waiting for a new SIM card to arrive. It is perfect for travel.

 Plus, since eSIMs are built into your device, there’s no risk of losing or damaging them, keeping your data safe and secure. However, eSIMs aren’t yet available on all devices or countries.

Not all carriers support eSIM cards. However, if you want one for traveling, you can always use a global eSIM provider, Holafly, and get an eSIM for your destination.

Using multiple SIM cards on a phone

Many modern smartphones offer dual SIM, allowing you to use two SIM cards at the same time. This is useful for keeping work and personal numbers on the same phone, using a local SIM for data while traveling, and cutting costs by using one SIM for data and the other for calls and texts.

Here’s how it works:

1. Dual SIM Phones: These phones have two SIM card slots. You can insert and manage both SIM cards through the phone’s settings, assigning specific functions (like calls or data) to each SIM.

      slot for two sim cards
      Source: Canva

      When you add a second SIM card to your phone, you will have to choose which SIM card you’ll be using for calls, texts, and data. For example, you can use SIM 1 for calls and SIM 2 for data within the same device.

      2. eSIM and Physical SIM Card Combo: Some newer phones offer one physical SIM slot and an embedded SIM. This allows you to activate the eSIM with a mobile plan while using the physical SIM slot for another plan.

      phone with both sim and esim
      Source: Holafly

      This works pretty similar to having two SIM cards in the same device. In many ways, the eSIM card will be used same as the SIM card. Today, you can choose from a wide variety of eSIM cards that are data-only, so you can keep your SMS messages and calls on your primary SIM and use your eSIM for staying connected to the internet.

      3. SIM Card Management: In your phone’s settings, you can choose which SIMcard  to use for calls, messages, and data. You can even set different ringtones for each SIM to distinguish between them easily.

      Where to get a SIM Card?

      Nowadays, it’s simple to get a SIM card. You just have to pick the mobile cellphone company or an authorized point of sale near your location and request the SIM Card. Each country has a list of mobile operators, such as Orange, Vodafone, Movistar, Claro, Telcel, AT&T, Verizon, T-Mobile, and many others.

      You can also get a SIM Card from an internet provider or e-commerce store such as examples are Amazon, eBay, and MercadoLibre.

      Different mobile operators
      Different mobile operators. Source:

      Summary: What is a SIM card?

      A SIM card (Subscriber Identity Module) is a small, removable chip in mobile devices that stores data for connecting to a mobile network. It is what enables users to use their mobile device to make calls, send SMS, and access mobile data. 

      Over time, SIM cards have evolved from the larger Mini SIM to the smaller Micro SIM and Nano SIM. The latest advancement is the eSIM, a digital SIM embedded in devices, eliminating the need for physical cards. 

      eSIMs make it easy to download and switch mobile plans directly on their phones, though not all devices or carriers support them yet. You can get an eSIM from a local or international service provider.

      As technology progresses, the traditional SIM card’s role continues to evolve, offering more convenience and security. Whether using a physical SIM or an eSIM, understanding how these small chips work and their capabilities can significantly enhance how we manage our mobile communications. Getting SIM cards for mobile devices is quick and easy for those looking to stay connected. They are available from mobile carriers, authorized retailers, and online stores.

      Whether you’re a frequent traveler, a busy professional, or someone who values convenience, understanding the latest in SIM technology can help you make the most of your mobile device.

      What is a SIM card: Frequently Asked Questions

      Can a phone work without a SIM card?

      Some functions of your phone are available even if you don’t have a SIM card, but not all. Without a SIM card, the phone cannot make or receive calls, send or receive text messages, or use mobile data. However, it can still connect to Wi-Fi, allowing access to internet services, apps, and web browsing. This means you’d be able to use WiFi to log into social media and make VoIP calls through apps like WhatsApp.

      What happens if you take out your SIM card and put it in another phone?

      When you move your SIM card to another phone, your phone number, network connection, and any contacts or SMS stored on the SIM transfer to the new phone. Apps and personal data do not transfer, and the new phone must be compatible with your carrier’s network. However, keep in mind that not all data from your mobile phone is necessarily stored on your SIM. Some of it is in your mobile phone memory. Don’t expect to clone your phone simply by switching cards.

      Does a SIM card have a phone number?

      Yes, a SIM card typically has a phone number. This phone number is assigned by the mobile network operator when the SIM card is activated. The SIM card stores various information necessary for connecting to the mobile network and allows you to use your phone number as intended.

      How many SIM cards can be used on a cellphone?

      The answer depends on the cellphone model and its compatibility with the different formats of SIM technology. Some devices accept SIM, Dual SIM, eSIM and soon iSIM.