What is a SIM card? Everything you need to know (2024)

What is a SIM card? Everything you need to know (1)

Edgar Cervantes / Android Authority

SIM cards have been an integral yet forgettable aspect of owning a smartphone over the past few decades. For most users, the SIM card defines the basic commercial relationship they have with their carrier. However, as we slowly inch forward, SIM cards are increasingly on their way out, losing importance to eSIM, and newer iPhones don’t even have a physical SIM card in the USA. If you ever wondered what exactly is a sim card, and whether you still need one in 2024, we’re here to answer your queries.


A SIM card is a small chip that you insert into your phone and allows you to connect to your carrier's network. You can then make phone calls, send messages, and use mobile data through your carrier. SIM cards are still very popular in 2024, but thanks to eSIMs and the proliferation of Wi-Fi, a physical SIM card is no longer as necessary as it once was.


  • What is a SIM card?
  • What does a SIM card do?
  • How does a SIM card work?
  • Can a phone work without a SIM card?
  • How many types of SIM cards are there?
  • SIM vs eSIM: What's the difference?
  • How to insert a SIM card into a phone
  • Can I use two SIM cards on a phone?

What is a SIM card?

What is a SIM card? Everything you need to know (2)

Edgar Cervantes / Android Authority

SIM stands for “Subscriber Identity Module,” and a SIM card is essentially the card that holds your subscriber identity data. It’s the small chip-like card that you insert into your phone before you can begin using it. A SIM is essentially an integrated circuit that holds the data needed to confirm your subscription identity to a carrier’s network.

Think of it like this: A SIM card is similar to a passport in some ways. A passport indicates that you are a citizen of a particular country, and through it, you can enjoy the rights available to citizens of that country. A SIM card is essentially the passport to your carrier, denoting that you are a subscriber to the carrier’s services. The carrier is reassured of your status as a subscriber, and lets you enjoy the services it offers.

A SIM identifies you as a subscriber of a network.

“SIM” and “SIM card” are often used interchangeably. SIM refers to the identification technology as a whole, while SIM card refers to the plastic card that holds the gold-colored electronic contacts, which enables SIM tech.

What does a SIM card do?

What is a SIM card? Everything you need to know (3)

The primary purpose of a SIM card is to identify you as a network subscriber. It’s the key that lets you into your carrier’s network. There’s more nuance and details at play here, but we’ll keep it simple for this explanation.

For example, if AT&T has a good network in your region and you want to take advantage of it, you’d subscribe to AT&T’s network, and the carrier would then issue you a SIM card. You can then physically insert this SIM card into your smartphone to access AT&T’s network to make phone calls, send SMS and MMS, and access mobile data.

Without the SIM card in place, your phone wouldn’t know what network it needs to connect to, and the network wouldn’t know that the phone in your hand belongs to a subscriber. You won’t get a phone signal in your phone without a SIM, so you will not be able to make phone calls. The only exception would be emergency calls to 911, which you can place even without a SIM card.

How does a SIM card work?

A SIM stores several crucial pieces of data, including:

  • ICCID (Integrated Circuit Card Identification Number): This is a unique 18-22 digit code that is used to identify the physical SIM card itself. In common parlance, it is also called the SIM card number, but do not confuse it with your mobile number.
  • IMSI (International Mobile Subscriber Identity Number): This is a 14-15 digit code that forms the basis of identification for the subscriber.
  • Data related to security authentication such as the authentication key, LAI (Local Area Identity), and more.
  • Carrier-specific data such as SPN (Service Provider Name), SDN (Service Dialling Numbers), network identifiers, and more.

Here’s a simplified flow of what happens when you insert a SIM card into your phone:

  1. Your phone pulls the IMSI contained within the SIM card.
  2. The phone identifies the mobile operator from the code and contacts it.
  3. The phone passes the IMSI to the mobile operator for authentication.
  4. The mobile operator searches within its database for the IMSI.
  5. Upon successful location, the authentication key associated with the IMSI is also found. This is then utilized to help confirm the identity and authenticity of the SIM. There’s a fair bit of back and forth between the phone and the network operator in the authentication process, but these technicalities are outside the scope of this article.
  6. Once authenticated, the mobile operator grants the phone access to its network.

The SIM is the most crucial piece in this whole puzzle, namely for the data it contains. The physical card is less important, and that is why alternatives like eSIMs have evolved to provide similar functionality.

Can a phone work without a SIM card?

What is a SIM card? Everything you need to know (5)

Ryan Haines / Android Authority

The absence of a SIM doesn’t actually stop you from using a phone or connecting to the internet on a phone. Phones can be enjoyed without a SIM in place, with the caveat that certain network functions will understandably not work. For instance, you can still connect to a Wi-Fi network and access the vast majority of internet functions on your phone without inserting a SIM card. You can not, however, make phone calls, send SMS-based text messages, or connect to a carrier’s network.

As for the physical SIM card, solutions like eSIM and iSIM mean you don’t need the physical card anymore, while still allowing access to all relevant SIM information to ensure a successful connection to a mobile network.

How many types of SIM cards are there?

There are four main types of SIM cards on the basis of their physical size, although only the smallest retain relevancy in current times. Beyond them, there are two other types of SIMs that people should be aware of.

Full-size SIM, mini-SIM, micro-SIM

What is a SIM card? Everything you need to know (6)

By Justin Ormont - Own work, CC BY-SA 3.0, Link

The full-size SIM was the first format of SIM cards, issued all the way back in the 1990s. It was the size of a credit card, although the total area of the gold-colored contacts was the same as we see now.

Mini-SIM and micro-SIMs were introduced later on, with the intention to reduce the size of the plastic holding the gold-colored contacts and allow for smaller mobile handsets. As mentioned, they retained the same area for the gold-colored contacts as the full-size SIM. In the transition phases between two sizes, mobile operators would issue the bigger size SIM, but offer an easy way to “break” the SIM into smaller sizes.


This is the current size of SIM cards that most popular smartphones accept. It’s the smallest size possible while maintaining backward compatibility, as the size of the card is practically just the gold-colored contacts and a very thin layer of plastic.

However, phone manufacturers find this small size enough of a liability to consider removing it completely. For instance, the iPhone 14 series completely removed the SIM card slot and opted for an eSIM-only solution in the US. The same applies to the iPhone 15 series.

eSIM (Embedded SIM)

Advancements in technology have allowed SIMs to be programmed remotely, allowing the SIM to take the form of a chip built into your phone. This means eSIM users no longer need to physically insert or remove SIM cards. The eSIM configuration profile can be installed onto a phone by the network (usually by providing a QR code that the subscriber can scan).

eSIM support started off rather slow but has built up steam in the past few years. As already mentioned, Apple removed the SIM slot entirely on newer iPhones sold in the US. This skyrocketed eSIM adoption in the region, as well as across the globe. For better or for worse, most phones and most popular carriers provide eSIM options nowadays.

iSIM (Integrated SIM)

iSIM shrinks the SIM down even beyond what eSIM could achieve. It integrates the SIM directly into the modem chip or the SoC of the phone. So you’re no longer reserving the (relatively) larger space that an eSIM needed. While this technology has been slow to pick up on smartphones, it has great scope in IoT devices.

SIM vs eSIM: What’s the difference?

What is a SIM card? Everything you need to know (7)

Edgar Cervantes / Android Authority

When trying to understand the difference between a SIM and an eSIM, note that the SIM within this context usually refers to a nano-SIM card. It does not refer to the SIM technology per se — as an eSIM also includes SIM technology. It’s just not available physically for the user to access.

SIM cards are different from eSIMs primarily in their physicality: you can touch a SIM card, but you cannot touch an eSIM (unless you rip the phone open). The eSIM resides within the phone and isn’t user-accessible. Physical SIM cards exist outside of the phone and are inserted into the SIM slot present on the phone. The most significant advantage that eSIM brings is space saving for phone manufacturers, as they can now eliminate the SIM slot without crippling network functions on the device.

eSIMs are also theoretically easier to transfer across devices. If you lose your phone, you can be issued a new eSIM on your new phone without needing to visit your carrier. You can also quickly and easily swap between different carriers, by just enabling the relevant carrier profile.

Not all operators and all phones support eSIMs.

However, network operators often have convoluted processes for issuing eSIM configurations, so there can be pain points coming in from your carrier and their outdated workflows. In comparison, you can usually swap a physical SIM card from one phone to another within a few easy seconds. If you lose your phone, your carrier will have to ship you a new SIM card (which can take days to arrive), or you will have to walk into a carrier store yourself. To swap between carriers, you do have to remove the SIM and pop a new one in — but it’s a very reliable method that doesn’t fail.

Also, note that not all operators and all phones support eSIMs seamlessly, while a more blanket statement can be made for physical SIM cards. Many large operators continue to support both eSIMs and physical SIMs, and most recent phones come with support for both. If your phone is eSIM-only, then international roaming will also be a pain point, as you can’t just pop in a new local SIM card when traveling abroad. It doesn’t help that, outside the US, eSIM support isn’t as common, and sometimes it’s only available on larger, more expensive carriers.

How to insert a SIM card into a phone

What is a SIM card? Everything you need to know (8)

Robert Triggs / Android Authority

Despite the wide diversity of smartphones, the process of inserting and removing a SIM card is largely the same.

  1. Locate the SIM card slot. It is usually present on the phone’s mid-frame and looks like a closed cupboard drawer. Look at the image above for an example.
  2. Locate the pinhole near the SIM card slot.
  3. Use the SIM ejector tool that came with your phone, or a small paperclip, to press into the pinhole. This will eject the SIM card tray.
  4. You can now place the SIM card in the SIM card tray.
  5. There will likely be markings present on the SIM slot tray to indicate which side of the tray should be facing the screen. Insert the tray back in that orientation.

Warning: Note that phones also have microphone holes that look similar to the pin holes for SIM removal. These mic holes are usually present on the top and bottom edges of the mid-frame. Inserting the SIM removal tool into the mic holes could potentially damage the microphone if you apply excessive pressure.

Can I use two SIM cards on a phone?

What is a SIM card? Everything you need to know (9)

Most recent smartphones offer dual-SIM connectivity thanks to eSIM support. If your phone supports eSIM, it likely also has a physical SIM card slot. Manufacturers now mostly allow you to enable one eSIM profile and one physical SIM card, allowing you to have two lines in a single device. Some phones even allow you to program two eSIM profiles at a time.

Additionally, some devices come with actual dual nano-SIM card slots. These usually have a longer tray that accommodates two SIM cards (as shown in the picture above), or it could accommodate two SIM cards behind each other with their gold contacts facing outwards.

However, physical dual-SIM support can be a bit more hit-and-miss for US residents, as its usage is not very popular in this region as compared to the rest of the world. You can explore these dual-SIM Android smartphones if you are looking to run two SIMs on a single device.


SIM cards are backwards compatible through the use of adapters. You can also use punches to cut out a smaller SIM card for a larger-sized card. However, your carrier will easily issue you a newer SIM card for a nominal fee, and we recommend that you choose that option for the additional security and benefits present in newer, higher-storage SIM cards. Of course, eSIMs are an exception, as you can’t remove them.

No, you do not need a new SIM card to upgrade from 4G to 5G. The SIM upgrade was needed during the upgrade from 3G to 4G as the older SIM lacked certain capabilities. This is not the case for switching from 4G to 5G. However, ensure that your phone supports 5G, and that your carrier offers 5G in your region.

SIM cards do not usually go bad, but they can stop working due to poor maintenance and habits. If you remove them very frequently, the contact points could get scraped and fail. They can also fail due to corrosion and water damage. Bending a SIM card will also destroy the contact lines and cause failure.

Yes, SIM cards can be swapped between devices, as long as they are the same size. If sizes mismatch, you may need an adaptor or a punch, as the case may be. Now, if you’re using eSIM technology, there is sadly no easy way to transfer eSIM profiles between different manufacturers yet. If you need some help with this, we have a guide for transferring eSIMs between Android phones.

It depends. On iPhone 13 and older, you can easily use a SIM card from an Android phone by simply popping it into the SIM slot. However, the newer iPhone 14 series and iPhone 15 series are eSIM-only in the USA, so you will need to migrate your eSIM configuration profile. Sadly, there is no easy way to do this, but we have a guide to help you transfer eSIMs to your iPhone, if you need some help.

iPhone 13 and older iPhones come with SIM card support. However, iPhone 14 series devices sold in the US does not support a SIM card as the series is eSIM-only. The same applies to the iPhone 15 series. iPhones sold outside the US still retain SIM card support, though.

While it is technically possible to use a SIM card in an eSIM-only iPhone, we do not recommend users do so. The process involves disassembly and complicated motherboard resoldering, and thus, is best avoided by most people.

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