What Is a SIM Card? 8 Things You Need to Know

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What Is a SIM Card? 8 Things You Need to Know

By Simon Batt

Published 10 hours ago

SIM cards are an important part of your phone, but what do they do and how do you use them?

Image Credit: fortton/Shutterstock.com

To use your phone as an actual phone, you need to put a SIM card into it first. But what's a SIM card, why are they so important, and what happens if you use a phone without a SIM card?

Let's explore the world of SIM cards and how they help us connect.

1. What Does "SIM" Stand For?

Let's check off the basics first. The word "SIM" stands for "subscriber identity module," and its name alone tells you a lot about what a SIM card does. It's your miniature passport for using cellular data and identifies who you are on the network.

2. What Does a SIM Card Do?

You don't need a SIM card for a phone to turn on. You can test this yourself by removing the SIM card from your phone and booting it up. It should turn on just fine.

However, you'll notice that your phone won't get a signal. This is because your SIM card is responsible for holding all of the information required to connect you to the cellular network.

This includes the network you're subscribed to, your phone number, and what data you can use on it (3G, 4G, and 5G). As such, if you remove the SIM card, you remove the information the phone needs to connect to the network—hence why you get no signal.

You can still use non-cellular forms of data without a SIM card, such as Wi-Fi. However, features such as calls over Wi-Fi still require a SIM card, because the data is eventually handled by your network provider down the line.

Related: What Is a SIM Card and What Does It Do?

3. SIM Cards Come in Different Sizes

The different SIM sizesImage Credit: Crystal Eye Studio/Shutterstock.com

SIM cards in three different sizes; Nano, Micro, and Standard. The size that your phone accepts will depend on the phone's make, size, and recency. As such, when you want to get a new SIM card, you need to figure out which size your phone likes and order one of the correct sizes.

Fortunately, most cellular carriers have handled this issue very well. When you order a SIM card from a popular provider, they'll give you a Standard-sized one. However, the SIM card will have perforations inside of it that allow you to transform it into its Micro or Nano forms by tearing parts off of it.

4. Some Phones Let You Use Two SIM Cards at Once

A dual-SIM phoneImage Credit: Boris Sosnovyy/Shutterstock.com

You may have heard of phones being "dual SIM." This feature is a particularly useful one to have if you want to manage more than one phone number.

As you might expect from the name, a dual SIM phone allows you to put two SIM cards into one phone. This lets you manage two different phone numbers—one per SIM—on a single phone.

This is really handy if you want to separate your friend and family phone number from your work phone number. If you install both your work and leisure SIM cards into a dual-SIM phone, you'll receive calls to both numbers from one phone.

This also means you can call people from either number. Usually, this is handled when you try to call someone, as your phone will then ask you which SIM card you'd like to use to make the call.

And yes, this does mean you can benefit from two networks at once. For example, if you install two SIM cards from different carriers, SIM 1 may get a strong signal where SIM 2 can't, and vice versa.

Related: What Are Dual SIM Phones? Do You Really Need One?

5. SIM Cards Can Store (Some) Data

SIM cards can store a little bit of data. However, before you try to cram your photos onto it, SIM cards can only store up to about 256kb of data, so you can't squeeze a lot onto it.

What you can fit on it are contacts and SMS messages. This is handy when you're changing phones, as you can bring all your contacts with you without needing to re-add everyone.

If you're interested in taking a peek within the card, there are apps that help you manage your SIM card's data. You can use these to browse all the information stored within, which is handy if you're planning to upgrade to a new phone.

Speaking of changing phones...

6. SIM Cards Aren't Tied to One Phone

When you put a SIM card into one phone, it doesn't attach itself to that phone forever. You can take the SIM card out of the phone and put it in a new one, such as when you're upgrading.

There are a few caveats for this. For instance, if your new phone requires a different sized SIM card than your current one, you may need to modify your SIM card or order a new one. And, unfortunately, some phones just won't accept your SIM card at all...

7. You Can Only Use a Specific Carrier's SIM Card on a Locked Phone

When getting into the world of SIM cards, you need to get acquainted with two terms: "locked" and "unlocked." These two terms dictate which SIM cards you can use on a phone.

A "locked" phone can only use SIM cards from specific carriers. You'll usually see locked phones appear when a network carrier teams up with a device manufacturer to give their customers exclusive access to a specific phone. If you insert an incompatible SIM card into a locked phone, it won't work.

On the other hand, an "unlocked" phone allows SIM cards from any provider. This can be a phone created by a manufacturer who does not have an allegiance with any specific network carrier. Mobile phone shops and repair places can also "unlock" a previously locked phone so it accepts any SIM card. They usually sell for more than locked phones.

It's important to note that these terms are different than when you "unlock" your phone's security using a PIN code, pattern, or biometric scan. As such, if you see a phone advertised on the market as "unlocked," it doesn't mean it doesn't come with a lock screen!

Related: How to SIM Unlock Your Android Smartphone or Tablet

8. You Can Likely Use Your SIM Card Abroad (but You May Not Want To)

SIMs from different countriesImage Credit: Oleksiy Mark/Shutterstock.com

Some data providers allow what's called "data roaming" when you go overseas. This is when your cellular company strikes a deal with other companies abroad so that your SIM card can use their network.

However, before you rush off on holiday with your phone in your pocket, double-check your carrier's roaming charges. Some companies will add a pretty hefty price on using data abroad, meaning you can't use your phone as you would back home. And some countries may be more expensive to roam within than others.

If you want to save some money, it may be worth getting a SIM card when you arrive at your destination. That way, you'll pay the same rate as the citizens do.

You'll have a different number as a result, but it will make communication within the country a lot easier. Plus, if you have a dual-SIM, you can keep your current SIM in case someone wants to talk to you, and add the foreign SIM as your second one.

SIM Cards Made Simple

SIM cards can be confusing at first, but they're not as complex as you may first think. Now you know what SIM cards do, the sizes they come in, and how to use them.

About The Author


Simon Batt (701 Articles Published)

A Computer Science BSc graduate with a deep passion for all things security. After working for an indie game studio, he found his passion for writing and decided to use his skill set to write about all things tech.

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