SIM card

The Essential Guide to SIM Cards

A mobile phone needs three crucial components to connect to a network: the device, the battery, and a SIM card. 

In this guide, you can find out why a SIM card is so important, what it does inside your phone, how to swap it, and more!

What is a SIM card?

A SIM card is a small, plastic card that is placed inside your phone. It stands for “Subscriber Identity Module”, and each card has a unique identification number which networks can use to find your account information.

It connects you to a phone network so you can make and receive calls, send and receive text messages, and use mobile data. A SIM card also stores and retains your data, so when you swap devices, you can insert the same card and restore your information. 

What is stored on a SIM card?

Your SIM card stores your smartphone identity, which includes:

  • GPS location
  • Phone number
  • Network data
  • Contacts
  • Messages
  • Billing information
  • Your PIN

Can I use my phone without a SIM?

Technically, yes. But practically? No.

If you don’t have a SIM card in your phone, you will still be able to connect to Wi-Fi and access the internet. So you’ll still be able to use most of your apps, like messaging on social media, playing games, and accessing your music apps

However, without a SIM card, you can’t make or receive phone calls, send or receive text messages, or connect to mobile data.

Which size SIM do I need?

Physical SIM cards come in three sizes: Standard, Micro, and Nano. 

The chip, which is the part that stores your data, is the same size in all three, so their changing width and length is just down to the surrounding plastic. 

  • Standard SIM: This is the largest SIM (15mm x 25mm) and is typically used in phones from the 90s and early 2000s. It was introduced in 1996, but is starting to fade out of use now due to its large size and the evolution of thinner phones.
  • Micro SIM: This medium-sized SIM (12mm x 15mm) was introduced in 2003. Like the Standard version, this SIM is also used less frequently nowadays. A Micro SIM is usually compatible with phones that are too new for the Standard SIM, but not necessarily new enough to fit the Nano SIM. We’d place these phones at around 5+ years old.
  • Nano SIM: The smallest of the three (8.8mm x 12.3mm), the Nano SIM was launched in 2012. With barely any plastic surrounding the chip, this may be the smallest we can shrink SIM cards to, unless we find a way to downsize the chip itself. If you’re buying a new phone, chances are you’ll need a Nano SIM, as they are the most widely used now.

SIM cards have gotten smaller so more space can be utilised inside the phone, so bigger batteries or more sophisticated hardware can fill the inside of the phone instead. 

Find out which size SIM card you need for your device. 

What about eSIMs?

The newest introduction to SIM card technology is the eSIM

An eSIM is an electronic SIM card which acts as a reprogrammable chip that is embedded in your device using a QR code. It cannot be removed or swapped out as it is only digital, rather than a physical card you can hold. 

eSIMs can be digitally rewritten, which massively simplifies and speeds up the process of switching networks. They remove the hassle of deliveries, phone calls, swapping SIMs, and PAC codes.

As great as they sound, they’re a very new type of SIM, and you’ll only find them in a limited number of devices.

How do I take out my SIM card and insert a new one?

If you’re planning to switch networks, you’ll need to swap your SIM card too. Taking out your old SIM and inserting a new one is easy with our step by step video guide:

Watch our video

For Apple

  1. Locate the SIM card tray on the side of your iPhone
  2. If you kept your packaging, you can use the SIM-eject tool (or a paperclip) to pop the tray open by pressing the point into the little hole
  3. Pull the tray out and remove your current SIM card
  4. Place your new SIM card into the tray. It will only fit in one way so make sure you align the shape up properly
  5. Insert the tray back into your iPhone and press it shut

After your new SIM has been inserted, you may get a prompt to restore any data held on your SIM. 

For Android

  1. Locate the SIM card tray either under the battery or along the side of your phone. If your Android has a removable back plate or removable battery, the SIM slot can usually be found under there
  2. If your SIM tray is on the back, slide it open by lifting it up. If your SIM tray is along the side of your phone, use an ejector tool or paperclip to pop it open by pressing the tool into the little hole
  3. Remove your current SIM card
  4. Place your new SIM card into the tray, aligning the shape up properly to fit inside
  5. Close the lid or push the tray back into the side of your phone, and press it down to shut it firmly

Similar to iPhones, you may get a prompt to restore any data you held on your SIM. 

SIM card FAQs

If you already know what question you want answering, use the links below to quickly skip to what you need:

Can my SIM card be tracked?

The police can track a SIM card using cell tower masts, but this is only for criminal investigations. 

You can personally use the ‘Find My Device’ or ‘Find My iPhone’ apps to locate your phone if it has been lost or stolen. These services work by tracing your GPS signal so you can lock or erase your phone’s data remotely. 

Does a SIM card have memory?

SIM cards usually have around 8KB to 256KB of memory, which is enough to save your contacts and text messages for transferring across devices.

The rest of your information is usually saved on the internal memory of your phone, an SD card, or online through cloud storage.

Can a SIM card be used to stop roaming charges when I travel abroad?

Your SIM card will be associated with a network, so if you are with a UK mobile provider, you will adhere to their roaming charges when abroad. 

The ‘Roam Like at Home’ legislation was passed in 2017 to allow residents of EU member states to travel to other member states without being charged extra on their data usage, texts, and calls. 

Since the UK has left the EU, this protection is not guaranteed. Individual network providers will decide if they want to set roaming charges or not. 

With Asda mobile, you can roam like you’re at home in 36 European destinations. 

How secure is my SIM card?

SIM cards are mostly very safe, but you can activate a SIM PIN if you’re concerned. This PIN would ensure that your SIM couldn't be used without entering the correct code.

SIM cloning is technically possible, but extremely rare, as someone would need to remove your SIM card and place it into a special cloning machine. 

Can I have two SIM cards with the same number?

You cannot have two SIM cards with the same number. 

However, dual-SIM connectivity allows you to insert two different SIM cards into the same device to have two different phone numbers. This means you could have separate personal and work numbers on the same device.

Can my SIM create a personal hotspot?

You can create a mobile hotspot with most networks, including Asda mobile, to allow you to connect other devices to the internet. 

Can I keep the same SIM when switching networks?

SIM cards are locked to their current network, so you’ll unfortunately have to switch SIM cards when switching providers. The exception is eSIMs, which can be reprogrammed with a QR code. 

If you swap SIMs, you can also easily transfer your current number over.

Can you use a mobile SIM in a tablet?

Yes, you can usually use a mobile SIM card in a compatible tablet. Not all devices will be able to access a network signal like a smartphone though, so you’ll have to check with the manufacturer.

How long do SIM cards stay active? Do they expire?

On most networks, your SIM card and data will remain active as long as you make a chargeable activity every six months, at least. This could include making a phone call, sending a text message, or turning your mobile data on to access the internet whilst out and about.

If you don’t use any of these features, the credit associated with your SIM card (i.e. your bundle) will be deactivated.

However, some networks may cancel your SIM card sooner, if you don’t use it within 90 days. 

Technically, the SIM card itself doesn’t actually expire - it’s just the data, texts, and calls package on it that could be lost. 


Further reading

If you’re interested in learning more about SIM cards, feel free to read more of our blogs below:

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