What is a SIM card?

What is a SIM card?

We might not think about it too often, but the SIM card is a key part of how our smartphones function. Our experts are here to take you through what a SIM card is, what it contains, and how it helps you communicate with other people.

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What does a SIM card do?

The SIM (Subscriber Identity Module) card is a very small piece of plastic – typically 15 x 25mm – that has a computer chip on the top. SIM cards are needed to connect a smartphone to a mobile network, and do so via information stored on the chip. This information acts as a key, indicating to the network that you’re a subscribed user.

As well as connecting you to the Internet, a SIM card is needed for sending SMS messages and making calls.

How does a SIM card work?

A SIM card uses two pieces of information to authenticate your identity on a network: an International Mobile Subscriber Identity and an Integrated Circuit Card Identifier.

The IMSI is a 15-digit number that, alongside an authentication key, verifies you as a subscriber to the network. Once the IMSI and key match the network provider’s records, your phone will have access to the service.

The ICCID is an 18 to 22-digit number that is typically found on the back of the SIM card. Every SIM has its own ICCID, and it helps a service provider determine which network to connect you to.


What is stored on a SIM card?

A SIM card will often contain:

Your phone number
Text messages
Authorisation data
Security keys

What types of SIM card can you get?

Standard SIM – one of the most common SIM cards (15 x 25mm)
Micro SIM – typically found in older smartphones pre-2012 (12 x 15mm)
Nano SIM – typically found in smartphones post-2012 (12.3 x 8.8mm)
Dual SIM – a combination of two SIMs, usually one physical SIM and one eSIM. Useful for those who want two phone numbers attached to a device
eSIM – a small chip that’s pre-installed by the phone’s manufacturer. Replaces a physical SIM card


Advantages of SIM cards

Due to their small size, SIM cards are exceptionally portable and can easily be moved from one device to another. This is incredibly useful when changing phones, enabling you to transfer your number across and maintain most of your preferences.

SIM cards are also handy if your phone is out of battery power in an emergency. Simply take the SIM card out and put it in a different handset to make a call.

That is everything you need to know about SIM cards and what they do. If your phone has an eSIM rather than a physical SIM, we also have a useful guide on how to set one up.