What is a SIM card, and what does it do?

Key takeaways

• A SIM card is a small chip that slots into your phone to connect it to a mobile network, so you can make calls, send texts and use mobile data.
• SIM stands for Subscriber Identity Module. Its main job is to identify your number and account to the network, so the right calls, texts and data reach the right phone.
• Physical SIM cards come in three sizes (standard, micro and nano).
• An eSIM does the same job digitally, built into the phone itself, with no card to fit.

 

---

Ever stared at that tiny piece of plastic in a new phone box and wondered what it's for? Fair question. It's easy to assume it's just packaging, but that little chip is what makes your phone work.

What is a SIM card?

SIM stands for Subscriber Identity Module. It's a small, removable chip that fits into a tray on the side of a phone, and it tells your network who you are.

Think of it less as storage and more as an ID card for your phone. Without a SIM fitted (or an eSIM set up), your phone can usually still connect to Wi-Fi. But it won't be able to make calls, send texts or use mobile data.

What does a SIM card do?

A SIM card connects your phone to a mobile network and tells the network who you are. In practice, that means a handful of things:

• Connecting your phone to your network provider, so it has a signal
• Making and receiving calls and texts
• Using mobile data for browsing, streaming and apps
• Identifying your account and number to the network, so usage and billing line up correctly
• Storing a small amount of information, such as contacts, if you choose to save them there

Take the SIM out, and your phone stops being a phone. It's just a very nice camera.

What's stored on a SIM card?

Modern SIM cards don't hold much data. Usually, it’s just enough to identify your account to the network, plus a little space for contacts or texts if you choose to save them. Most people now keep contacts, photos and messages on their phone or in the cloud instead, so this storage is more of an old habit than a necessity.

You can also set a PIN on your SIM. It's a good habit, as it makes the SIM harder to use if it ends up in someone else's phone.

What sizes do SIM cards come in?

Physical SIM cards have shrunk over the years as phones have got slimmer. There are three sizes still in use:

SIM size	Size	Roughly used in
Standard	15 x 25 mm	Older phones, mostly before 2012
Micro	12 x 15 mm	Phones released around 2010 to 2014
Nano	8.8 x 12.3 mm	Most current smartphones

If you're not sure which one your phone takes, checking the model online is the quickest way to find out. We've covered how SIM sizes have changed over time in our guide to SIM card sizes.

Is an eSIM the same as a SIM card?

An eSIM (embedded SIM) does the same job as a physical SIM, just without the plastic. It's a chip built into the phone, and your network details are added to it digitally rather than by inserting a card.

The main practical difference is convenience. There's no card to lose or swap when you get a new phone. And some phones can run a physical SIM and an eSIM at the same time, which is handy if you want two numbers on one phone. Not every phone supports eSIM, though, so check your model first.

If you're curious how eSIM works, we've covered it in more detail in our eSIM guide.

How much does a SIM card cost?

With SMARTY, the SIM itself costs nothing. Every plan includes a free SIM, sent as a 3-in-1 card with standard, micro and nano sizes built in, or as an eSIM if your phone supports it. You only pay for the plan you choose. And because it rolls over monthly, nothing locks you in while you work out if it's right for you. Want a closer look? See what’s included across all our plans.

FAQs

1. What is a SIM card used for?

A SIM card connects a phone to a mobile network, allowing you to make calls, send texts and use mobile data. It also stores the account details that identify the phone to the network.

2. Does a SIM card come with a phone number?

Yes, a SIM card is normally issued with a phone number already assigned to it by the network, so inserting it into a compatible phone activates the number on that device.

3. What happens if a SIM card is lost or damaged?

A lost or damaged SIM card can usually be replaced by the network provider, often for a small fee, and the existing phone number can typically be transferred to the replacement SIM.