What is a SIM Card?

A SIM card, also called a subscriber identity module or subscriber identification module, is a small memory card that contains unique information that identifies it to a specific mobile network. This card allows subscribers to use their mobile devices to receive calls, send SMS messages, or connect to mobile internet services.

What Is a SIM Card Used For?

Some phones need a SIM card to identify the owner and communicate with a mobile network. For example, an iPhone on the Verizon network needs a SIM card so that Verizon knows who the phone belongs to and that they are paying for the subscription, and also so that certain features work.

This is important in resale situations, where a used smartphone is missing a SIM card. As such, you may be able to use the device's camera or Wi-Fi features, but you can't send texts, make calls, or connect to a carrier's mobile internet network.

Some SIM cards are mobile, which means if it is transferred to a new or upgraded phone, the phone number and carrier plan details transfer as well. Similarly, if the phone runs out of battery and you need to make a phone call, and you have a spare around, you can put the SIM card into the other phone and immediately use it.

The SIM card also contains a small amount of memory that can store up to 250 contacts, some SMS messages, and other information used by the carrier who supplied the card.

In many countries, SIM cards and devices are locked to the carrier the device is purchased from. This means that although a SIM card from a carrier works in any device sold by that same carrier, it doesn't work in a device that's sold by a different carrier. It is usually possible to unlock a cellphone with help from the carrier.

Does My Phone Need a SIM Card?

You might have heard the terms GSM and CDMA in relation to your smartphone. GSM phones use SIM cards while CDMA phones do not.

If you're on a CDMA network like Verizon Wireless, Virgin Mobile, or Sprint, your phone may have a SIM card or SIM card slot. This is most likely because the LTE standard requires it, or because the SIM slot can be used with foreign GSM networks. However, in these situations, the identification features are not stored on the SIM. This means if you have a new Verizon phone that you want to use, you cannot put your current SIM card into the phone and expect it to work. To do that, you have to activate the device from your Verizon account.

The SIM card on GSM phones can be swapped with other GSM phones. The phone will then work on the GSM network that the SIM is tied to, such as T-Mobile or AT&T. This means you can remove the SIM card in one GSM phone and put it in another and keep using your phone's data, phone number, and other services, without getting approval through the carrier like you have to when using Verizon, Virgin Mobile, or Sprint.

Originally, cellphones that used the CDMA network rather than the GSM network did not use a removable SIM card. Instead, the device contained the identifying numbers and other information. This meant that CDMA devices could not easily be switched from one carrier network to another, and could not be used in many countries outside the U.S.

More recently, CDMA phones have begun to feature a Removable User Identity Module (R-UIM). This card looks almost identical to a SIM card and works in most GSM devices.

What Does a SIM Card Look Like?

A SIM card looks like a small piece of plastic. The important part is a small integrated chip that is read by the mobile device it is inserted into. The chip contains a unique identification number, the phone number, and other data specific to the user.

The first SIM cards were roughly the size of a credit card and were the same shape. Now, both Mini and Micro-SIM cards feature a cut-off corner to prevent incorrect insertion into the phone or tablet.

Here are the dimensions of the different types of SIM cards:

• Full SIM: 85 mm x 53 mm
• Mini-SIM: 25 mm x 15 mm
• Micro-SIM: 15 mm x 12 mm
• Nano-SIM: 12.3 mm x 8.8 mm
• Embedded SIM: 6 mm x 5 mm

If you have an iPhone 5 or later, the phone uses a Nano-SIM. The iPhone 4 and 4S use the larger Micro-SIM card.

Samsung Galaxy S4 and S5 phones use Micro-SIM cards while the Nano-SIM is necessary for Samsung Galaxy S6 and S7 devices.

Despite differences in size, all SIM cards contain the same types of identifying numbers and information on the chip. Different cards contain different amounts of memory space, but this has nothing to do with the physical size of the card. A Mini-SIM card can be converted into a Micro-SIM, as long as it's only the plastic surrounding the card that is physically cut out or removed.

Where Do I Get a SIM Card?

You can get a SIM card for your phone from the carrier that you subscribe to. This is usually done through customer service. For example, if you have a Verizon phone and need a Verizon SIM card, ask for one in a Verizon store or request a new one online when you add a phone to your account.

How Do I Remove or Insert a SIM Card?

The process of replacing a SIM card varies depending on the device. It might be stored behind the battery, which is only accessible through a panel at the back. However, some SIM cards are accessible on the side of the phone or mobile device.

The SIM card for your specific phone might be one where you pop it out of its slot with something sharp like a paperclip, but others might be easier to remove where you slide it out with your finger.

Frequently Asked Questions

• What is stored on a SIM card? SIM cards contain data specific to the user, such as their identity, phone number, contact lists, and text messages. 
• What is the difference between a SIM card and an SD card? While SIM cards store data related to cellular connectivity, Secure Digital (SD) cards store other information, such as pictures, music, and cell phone apps. If your images aren't displaying correctly, for example, it's probably due to an SD card issue, and not a SIM card issue.
• What is a Nano SIM card? A Nano SIM card is physically smaller than a traditional SIM card; however, the technology of a Nano SIM is identical to that of a larger or smaller SIM card. You can insert a Nano SIM card into any SIM card slot by attaching an adapter to the SIM card.
• What is a prepaid SIM card? A prepaid SIM card is pre-loaded with a set dollar amount, acting as a credit balance with the service provider. The provider charges against that amount for talk, text, and data use. Once the balance reaches zero, the provider discontinues service immediately.