What is a SIM Card?

In this guide we'll explain what a SIM Card is and how to install one

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What Does a SIM Card Do?

Michelle McLean

Michelle McLean

You might have stumbled upon the term SIM card, especially when the topic of BYOP plans are brought up. However, you may not exactly know what a SIM Card does beyond the fact that you should have one in your phone.

In that case, I have the answers for you. At the end of this article, you'll understand all there is to know about this SIM card.

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SIM Card Definition

Subscriber identity modules, or SIM cards, have actually been around for quite a long time - in fact, the first SIM card, which was 85.60 mm in length, was introduced back in 1991. Since then, SIM cards shrunk to more comfortable sizes. You can learn more about the history of the SIM card here.

However, what exactly is a SIM card? Well, this tiny form of personal identification, which can be slipped into a SIM card slot, helps you connect with your mobile phone provider and allows you to participate in its network. Without a SIM card, you cannot use these networks.

In other words, the SIM card lets your phone company know who you are and whether or not you have permission to access the network. The SIM card is also a nifty piece of security for your phone.

Another benefit the SIM card holds? It happens to store your contacts and messages, as well as some other data.

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Installing a SIM Card into Your Phone

Typically, the subscriber identity module is slipped into a SIM card tray inside your device but there are some devices that do not feature a SIM card tray, like the CDMA version of the iPhone 4.

If you are considering purchasing phone that does not feature a SIM card slot, then you should think about buying a SIM card reader, which can allow you to backup and save contact information, transfer contacts to another SIM card, and access information from another phone. To learn more about how to use a SIM card reader, you should refer to the eBay article.

Nevertheless, the subscriber identity module SIM may not always fit snugly into a card tray - or SIM slot - so you will need to purchase a compatible subscriber identity module SIM for the card tray if you're considering a SIM swap.

Swapping SIM Cards

So right now you might ask, "What in the world is a SIM swap?" Well, fortunately, I can tell you that a SIM swap is a process of replacing your existing SIM card with a new SIM card, all while shifting your phone number to the new SIM. Best of all, you can use any unactivated SIM card to perform the SIM swap.

So what else is there to know about the SIM card?

For one, the SIM card falls under the bracket of the smart card. A smart card is any pocket-sized card that features embedded integrated circuits. Smart cards often also provide personal identification information and authentication. Other examples of smart cards - besides the SIM card - are ATM cards, fuel cards, high-security identification cards, and more.

Furthermore and more specifically, this particular smart card sports its own serial number. However, this serial number is not your IMEI number. You can typically find your SIM serial number on the side of the card, which is 19 or 20 digits long, as well as specific information regarding your provider, location, and date of construction. The serial number is also known as the ICC-ID or Integrated Circuit Card ID.


The module SIM actually is found in GSM phones. GSM carriers typically place customer data onto a removable SIM card, unlike a CDMA-compatible phone.

So again, a GSM phone accepts a SIM card, meaning that a GSM phone can easily be verified onto a GSM network, thanks to a GSM SIM card. This is unlike CDMA carriers, who rely on network-based white lists to verify subscribers' data.

However, it should be noted that Verizon and Sprint - the two carriers that run on CDMA technology - allow users to switch phones and transfer data with SIM cards. But how, considering the fact that both run CDMA technology?

Well, the reason for this is because Sprint's and Verizon's SIM cards mainly exist to access the 4G LTE, or their LTE standard, networks. In other words, both companies rely on LTE SIM technology. LTE SIM data is just another network element that is taken into consideration in the mobile phone world.

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SIM Card Pin Code

The SIM card pin code isn't exactly essential, but you should consider having a pin code in the event that your SIM card is removed from your device. This way, no one else will be able to access your personal information.

Furthermore, if you happen to remove your SIM card and slip it into another device, you'll be asked for the SIM card pin code. Fortunately, if you've forgotten your card pin, you can always locate the pin code by contacting your service provider. This way, you'll be able to secure your card pin before you lock yourself out, which can take up to three attempts depending on the carrier.

If you didn't set this particular SIM number, then you should be aware that there may be default SIM card numbers associated with the provider. After you put in the default code, you will have the opportunity to change this SIM number, so when you do so, please remember your SIM card number.

SIM cards come in a variety of sizes

What is a PUK Code?

PUK, which stands for pin unlock code, is basically your safety net during a SIM lock.

For many carriers, when you transfer a SIM card from one phone to another, you'll have three attempts to unlock your SIM card with your SIM card pin code. However, after your three attempts are used, you'll be unable to use your SIM card - hence SIM lock. From there, you'll need to unlock your SIM card utilizing the unlock code or PUK.

After using this unlock code, you'll be forced to create a new PIN for your SIM card. Typically, you can find the SIM PUK numbers for your SIM card using your carrier's website or on the SIM card packaging. However, you can also contact your service provider via live chat or telephone.

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SIM Card Sizes

The SIM card comes in different sizes. Some phones are compatible with one type of size, while other mobile devices accept a different size. Fortunately, many service providers offer SIM cards with a cutout feature, meaning that you can choose the size you need for your SIM card. Here are the available SIM card sizes:

Standard SIM Card

The standard SIM, or mini-SIM, is actually the "second generation" of SIM cards. In other words, the standard SIM is referred to as 2FF (second form factor). The reason why this is the standard SIM is that carriers no longer use the full-sized first-generation SIM card. The measurements of the standard SIM is 25mm x 15mm x 0.76mm. Additionally, the standard SIM was introduced back in 1996.

Micro SIM Card

The micro-SIM, otherwise known as 3FF (third form factor), is the "third-generation" of SIM cards. The micro-SIM card measures 15mm x 12mm x 0.76mm. Micro SIM cards were introduced for the purpose of fitting into devices too small for the previous generation of SIM cards. Furthermore, the micro-SIM is backward-compatible, featuring the same assets and design as the mini-SIM.

Nano SIM Card

The Nano SIM is the "fourth generation" of SIM card, which has become quite standard in today's cell phone world. Also known as 4FF (fourth form factor), the Nano SIM measures 12.3mm x 8.8mm x 0.67. As an added tidbit, this Nano SIM card was unveiled during early 2012.

Which SIM Card Fits My Phone?

Many phones feature SIM cards, which allow you to maintain your personal subscriber data. However, the currently available phones come with different SIM card sizes. I will quickly highlight the available SIM card sizes for the most popular Samsung Galaxy phones, as well as the iPhone series.

Apple iPhone SE SIM Card Nano SIM
Apple iPhone 4 SIM Card Nano SIM
Apple iPhone 4S SIM Card Nano SIM
Apple iPhone 5 SIM Card Nano SIM
Apple iPhone 5S SIM Card Nano SIM
Apple iPhone 5c SIM Card Nano SIM
Apple iPhone 6 SIM Card Nano SIM
Apple iPhone 6 Plus SIM Card Nano SIM
Apple iPhone 6S SIM Card Nano SIM
Apple iPhone 6S Plus SIM Card Nano SIM
Apple iPhone 7 SIM Card Nano SIM
Apple iPhone 7 Plus SIM Card Nano SIM
Apple iPhone 8 SIM Card Nano SIM
Apple iPhone 8 Plus SIM Card Nano SIM
Apple iPhone X SIM Card Nano SIM
Apple iPhone XR SIM Card Nano SIM
Apple iPhone XS SIM Card Nano SIM
Apple iPhone XS Max SIM Card Nano SIM
Apple iPhone 11 SIM Card Nano SIM
Apple iPhone 11 Pro SIM Card Nano SIM
Apple iPhone 11 Pro Max SIM Card Nano SIM
Samsung Galaxy S5 SIM Card Nano SIM
Samsung Galaxy S6 SIM Card Nano SIM
Samsung Galaxy S6 Edge SIM Card Nano SIM
Samsung Galaxy S7 SIM Card Nano SIM
Samsung Galaxy S7 Edge SIM Card Nano SIM
Samsung Galaxy S8 SIM Card Nano SIM
Samsung Galaxy S8 Plus SIM Card Nano SIM
Samsung Galaxy S9 SIM Card Nano SIM
Samsung Galaxy S9 Plus SIM Card Nano SIM
Samsung Galaxy S10e SIM Card Nano SIM
Samsung Galaxy S10 SIM Card Nano SIM
Samsung Galaxy S10 Plus SIM Card Nano SIM
Samsung Galaxy Note 5 SIM Card Nano SIM
Samsung Galaxy Note 8 SIM Card Nano SIM
Samsung Galaxy Note 9 SIM Card Nano SIM
Samsung Galaxy Note 10 SIM Card Nano SIM
Samsung Galaxy Note 10 Plus SIM Card Nano SIM
Google Pixel SIM Card Nano SIM
Google Pixel XL SIM Card Nano SIM
Google Pixel 2 SIM Card Nano SIM
Google Pixel 2 XL SIM Card Nano SIM
Google Pixel 3 SIM Card Nano SIM
Google Pixel 3 XL SIM Card Nano SIM
Google Pixel 3a SIM Card Nano SIM
Google Pixel 3a XL SIM Card Nano SIM

Carrier SIM Cards

Fortunately, with a carrier SIM card, you'll be able to transfer your data to another phone and, better yet, the carrier network if you're considering switching to another carrier. Even CDMA networks such as Verizon and Sprint offer their own carrier SIM cards, but these cards are more for accessing the 4G LTE carrier networks.

Verizon SIM Cards

If you plan to switch to Verizon you'll be offered a SIM card if you're keeping your old device. What is unique to Verizon is its Verizon prepaid SIM card otherwise known as the 4G SIM Activation Kit. This prepaid option allows you to bring your own 4G Verizon phone to the network and activate a low-cost 4G LTE Plan without a contract. The plan is relatively inexpensive, but it's one size: unlimited talk and text plus 2GB of data.

However, you can add more data to your plan and carry over the data for up to 90 days. Also unfortunate is the phone compatibility selection, which is rather limited to older models of phone series.

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AT&T SIM Cards

AT&T offers two main types of SIM cards with its GSM-based network - one type for its post-paid service, and another type for its GoPhone (prepaid) service. The GoPhone SIM is widely compatible with feature phones, iPhone, certain Android and Windows Phones, and the BlackBerry Z10 and Q10.

Its post-paid SIM cards are also compatible with plenty of phones, including the iPhone.


T-Mobile SIM Cards

Like AT&T, the T-Mobile GSM network opens up a lot of doors to the SIM card world. The GSM T-Mobile SIM card typically comes inside the packaging of retail T-Mobile phones, but if you need to replace your SIM card, you will be forced to buy a SIM starter kit. If that is the case, you will need to backup all of your data (preferably onto a cloud) before you insert a new T-Mobile SIM card into your phone.

Additionally, the T-Mobile GSM network allows for easy switching of SIM cards between phones, as long as they share the same SIM card slot. So you should be able to connect to the GSM T-Mobile network without any difficulty, as long as you have a T-Mobile plan.

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Sprint SIM Cards

Does Sprint use SIM cards? The good news is that - despite Sprint hosting a CDMA network - Sprint introduced SIM card kits in 2016. You can purchase a 3-in-1 universal SIM card, which can be adjusted to fit any device. However, the device must be compatible with Sprint's bring your own device service.

Unfortunately, only domestic devices - not international devices - are supported by this service and the SIM card. However, you should also be aware that network band limitations in some devices can result in a subpar experience.

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Hopefully, you have learned all that there is to know about the SIM card. SIM cards go hand-in-hand with bring your own phone plans. If you plan on switching phone carriers but want to keep your current device, be sure to check out our article so you are prepared.

If you're looking for a new phone, you can compare cell phones side-by-side with our handy tool.

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