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

What is a SIM card?

A SIM card is the small chip that tells a mobile network who you are. So what is a SIM card for, day to day? It holds your account, so the network knows which number to ring, which plan to apply and who to bill. Take it out and the phone is still a phone. It just isn't yours, as far as the network is concerned.

SIM stands for Subscriber Identity Module. That expansion comes from ETSI's TS 102 221, the specification that defines the cards themselves. Google's Android help is plainer: your SIM card is "a removable chip that stores important info about your phone." Strictly, the info is about your subscription rather than the handset, which is why moving the card to another unlocked phone moves your number with it.

We put this together from ETSI's specification and from Apple's, Google's and Samsung's support pages, read on 9 November 2026.

What does a SIM card do?

Less than people think, and something more important than most of what a phone does.

When the phone switches on, it presents the identity on the SIM to the nearest mast. The network checks that identity against its own records, confirms the card is genuine, and only then lets the phone make calls, send texts or use data on your plan. Everything else, from the camera to the apps, has nothing to do with it.

So what does a SIM card store? Your subscription identity and the security details used to prove it. Older phones also saved contacts and texts to the card; modern phones keep those in the phone's own memory or in the cloud. Photos, apps and passwords have never lived on a SIM.

The chip itself is the same idea in every format. Standard, micro and nano are just different amounts of plastic around it: ETSI sets the nano at 12.3 mm by 8.8 mm, and it has been the size almost every phone takes for years. Apple's list puts every iPhone from the iPhone 5 onward on nano-SIM or eSIM, and Google says all Pixel phones can use nano SIM cards.

Is an eSIM a SIM card?

Yes, minus the card. Apple calls an eSIM "an industry-standard digital SIM built into your iPhone", and ETSI's specification already allows for the chip to be "embedded (e.g. soldered)" in the device rather than removable. You download the profile instead of pushing plastic into a tray. The job is identical.

Some phones no longer take a card at all. Apple says iPhone 14 models and later bought in the United States can only be activated with an eSIM, and Google says you can't use a physical SIM in a US Pixel 10, 10 Pro or 10 Pro XL. Our what is an eSIM guide covers how the download works.

Will a phone work without a SIM card?

Yes, on Wi-Fi. A phone with no SIM can still browse, run apps, send WhatsApp or iMessage messages, make video calls and use maps you downloaded earlier. What stops is everything tied to your number: ordinary calls, SMS, and mobile data once you leave the Wi-Fi.

Can a SIM-less phone reach emergency services? That depends on the country and the network, and we wouldn't rely on it.

Travellers sometimes pull the home SIM out to avoid roaming charges. It works, but it also stops bank security codes reaching you. Switching data roaming off for the home line usually does the job better, and our data roaming guide shows where the switch is. If you do want the card out, here's how to remove the SIM card from an iPhone.

What is a dual SIM phone?

Samsung's definition is about as short as it gets: "Dual SIM models allow you to use two SIM cards in one device." The two can be two plastic cards, one card and one eSIM, or two eSIMs, depending on the phone.

• iPhone: Apple says the iPhone XS, XS Max, XR or later can run two lines, and iPhone 13 models and later can use two eSIMs. Both lines take calls and messages, but the phone uses one cellular data network at a time.
• Pixel: Google says the Pixel 3a or later can use one physical SIM and one eSIM, and a Pixel 7 or later can run two eSIM profiles at once if your carrier allows it. "Only one SIM can be the default for data."
• Galaxy: Samsung describes single, dual and hybrid trays. On a hybrid tray the second slot takes either a SIM or a microSD card, never both. While one SIM is in a call, Samsung says, "the other card cannot be used to receive new calls at the same time."

For travel, dual SIM matters more than almost any other phone feature. Your home number stays live for texts on one line; a cheap local or travel plan carries the data on the other. Apple adds one condition: to use two carriers, the phone must be unlocked, unless both plans come from the same carrier.

What is a SIM card lock?

Two different things go by that name, and they get confused constantly.

The first is a SIM PIN. Apple describes it as a way to "require an identification code for phone calls and cellular-data usage", set on the SIM itself. Its warning is blunt: "Don't try to guess your SIM PIN." Too many wrong guesses can lock the card permanently, and your carrier may be able to give you a PUK code to undo it.

The second is a carrier lock, which is a lock on the phone, not the SIM. A locked phone accepts only its own carrier's cards. On an iPhone, check Settings, General, About, and look for "No SIM restrictions" next to Carrier Lock. Apple says only your carrier can unlock it. Fix this before a trip: a locked phone will refuse a foreign SIM at the airport counter.

What is a prepaid SIM card?

A prepaid SIM is paid for up front, with no contract and no monthly bill. You buy a bundle, use it, and top up or walk away. It's what most travellers buy abroad.

The rules for buying one vary a lot. Japan requires a passport or ID and an airport SIM costs around $12; the United States asks for no ID and a SIM at the airport typically costs around $20; in Thailand it's about $10. Those are the typical airport prices in our Japan, USA and Thailand guides. The cheapest eSIM table puts the local SIM price beside the best eSIM rate for every country we cover, and in a fair number of them the plastic card is the cheaper buy.