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The Essential Guide to SIM Cards

A mobile phone has three crucial components that are needed to connect to a network: the device itself, the battery, and a SIM card. In this guide, you’ll find out why a SIM card is so important and what it actually does inside your mobile phone. 

Last updated: 1st Feb 2021 at 4:56pm

What is a SIM card?

The first step to knowing why a SIM card is so important is to understand what this little card actually is. 

In essence, a SIM card allows you to connect to a phone network so that you can make and receive calls, send text messages, and use mobile data. SIM stands for “Subscriber Identity Module”; each card has a unique identification number on it, which networks use to associate your phone number, minutes, and other account information

This little smart card is placed inside your phone. Its job is to store and retain your network data, so that when you swap devices, you can plug the same SIM card in and immediately start using your phone. 

It stores your contacts, text messages, billing information and data usage, but the internal memory of your SD card remembers the other important information, like apps and photos. 


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 are still able to connect to Wi-Fi and access the internet. This means you can still go on Instagram, Facebook, and your emails, for example. 

You can also still use the majority of your apps on your phone, be it games you play, organisation tools you use, or music apps you listen to. 

You can’t, however, make or receive phone calls, send or receive text messages, or connect to mobile internet. A mobile phone isn’t really a phone without a SIM card, as you can’t perform any basic networking functions. 

If you’re looking for a new network, you can order a free SIM from us, or browse our SIM only deals


Which size SIM do I need?

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 width and length is actually just the plastic surrounding it. 

  • 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 size and the evolution of mobile phones
  • Micro SIM - the medium-sized SIM (12mm x 15mm) was introduced in 2003. Like the Standard SIM, this one is also used less frequently nowadays. A Micro SIM is usually inside phones that are too new for the Standard, but not necessarily new enough for the Nano (so 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, such as for bigger batteries. 

If you’re unsure what size SIM card is right for your model, you can read our guide to find the right one


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

If you’re switching networks, you’ll need to swap your SIM card too. Taking out your old SIM and inserting a new one is a simple process. Watch the video below to see how, or follow our step by step guide for Apple and Android phones: 

Watch our video


For Apple

  1. Locate the SIM card tray on the right-hand side of your iPhone
  2. Insert the SIM-eject tool that came with your phone (if you still have the packaging) or use a paperclip to pop the tray open. To do this, press the tool into the little hole beside the tray until it pops open slightly
  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 to the tray
  5. Insert the tray back into your iPhone and press it shut
  6. Now you have inserted your new SIM!


For Android

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



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Can my SIM card be tracked?

The police can track a SIM card using cell tower masts, but this is only for criminal investigations. Otherwise, you can personally use the ‘Find My Device’ or ‘Find My iPhone’ applications to locate your phone if it has been stolen or lost. These services work by tracing the GPS signal, and you can lock or erase your phone data using these apps. 


Does a SIM card have memory?

SIM cards do have around 64KB or 128KB of memory, which is just enough to save your contacts and text messages for when you transfer across to a new device. The rest of your information is usually saved on the internal memory of your phone (which can reach a whopping 250GB if you pay for it), an SD card, or online through Cloud services. 


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

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

Though the EU has implemented a “Roam Like at Home” policy where you won’t be charged more when using your phone in other member states, Brexit may change what is allowed and permitted here. 

Find out more about data roaming charges.


How secure is my SIM card?

SIM cards are quite safe. You can activate a SIM PIN if you’re concerned, which means your SIM can’t be used in your phone or any other phone without entering the correct code. SIM cloning is possible, but extremely rare, as someone would need to remove your card and place it into a special cloning machine. 


Can I have two SIM cards with the same number?

No, you cannot have two SIM cards with the same number. The only exception to this is an eSIM, which is built into some smartwatches to allow people to leave their phone behind. 


Can my SIM create a personal hotspot?

Yes, you can create a mobile hotspot with most networks (you definitely can with Asda Mobile), allowing you to connect other devices to the internet. 


Can I keep the same SIM when switching networks?

No you can't. If you are switching mobile networks, you'll receive a new SIM from your chosen provider. The good news is you can easily transfer your number over.

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