Ultimate Guide to SIM Cards

Your mobile phone has three components that enable it to connect to a network and make calls. These three pieces of hardware include the phone, its battery, and a SIM card. People like to swap their phones often, but they always keep the SIM card and move this from old phone to new.


Last updated: 21st Mar 2019 at 2:08pm

If you have ever wondered what your SIM actually does, then this is the guide that you have been looking for:

What does SIM mean?

Firstly, we are going to give some very important, top secret, impress at a party whilst standing in the kitchen information… SIM actually stands for something, sayyyy whattttttt…. “Subscriber Identity Module” - sort of makes sense doesn’t it, now take this piece of information and impress/utterly bore your friends & family.

Can I use my mobile phone without a SIM?

The answer to this question is both yes and no. Without a SIM card, you can still connect to a Wi-Fi network to connect to the internet. This will allow you to check your email, visit Instagram, and send messages through a service such as Messenger.

However, you cannot make calls over the network, receive a call, send a text or connect to the internet, when you are away from a Wi-Fi hotspot. In essence, without a SIM card, your phone has the features of a Wi-Fi only tablet.

It is the SIM card that gives you a telephone number to make calls and send texts. The SIM card also lets you connect to the internet (4G) when you are out and about.

Need a new SIM Card? Order a Free SIM from Asda Mobile.

Which size SIM do I need?

SIM cards come in three different sizes and two of these are now uncommon and are being phased out. The three SIMs available include:

  • Standard SIM – This is the largest SIM (25mm x 15mm) and is used in phones from the 90s and early 00s

  • Micro SIM – This medium-sized SIM (15mm x 12mm) was introduced in 2010 and is used in older smartphones such as the iPhone 4

  • Nano SIM – Introduced in late 2012, this is the smallest SIM (12.3mm x 8.8mm) and is usually used in phones released over the last 3-4 years, such as the iPhone 5 and above

In the past, many people used to cut down their standard SIM to the size of a micro SIM. We should point out here that you cannot cut down a micro SIM to the size of a nano-SIM because nano-SIMS are thinner.

All of these SIM sizes are supplied when you order a free SIM pack from Asda Mobile. When you receive your SIM pack simply see which SIM fits into your phones SIM slot.

Read our full guide to what SIM card you need.

Will my SIM work in another phone?

If you have brought your phone from a mobile network provider along as part of a deal including data, minutes and texts then it will most likely be locked. However, your SIM should work in any unlocked phone. If your SIM isn’t the right size, then you can contact your network for a new SIM card with the same telephone number.

I have a new SIM card; Where is my data?

Your SIM card can store your contacts and text messages, so if for example, you put your SIM in the latest iPhone, then this data will transfer across. If you need a new SIM card then you can save your contacts and messages to your Google account or Apple iCloud account.

This backup feature is automatic on the latest phones and can be used to restore your social media apps, games, photos, and anything else that may be stored on your old phone.

Can my SIM card be tracked?

The police can track a SIM card using cell tower masts if the phone is left on, although this form of tracking is reserved for serious crime investigations. If you lose your phone or think that it has been stolen then you can use the ‘Find My Device’ or ‘Find My iPhone’ applications to locate your phone. This service locates the phone through its GPS signal and if your phone has been stolen then you can protect your data by locking the phone or erasing its data, using this service.

Does a SIM card have memory?

SIM cards usually have 64KB or 128KB of memory, which is just large enough to save the details of 250 contacts and essential data such as your service provider name. Everything else that you want to save will be saved on the memory of your phone or online with a Cloud service such as Google Photos.

Can a SIM stop roaming charges when I travel abroad?

The new EU Roam like at Home rules let you use your phone like at home in any EU country, for the same price. Any calls you make over your allowance will all be charged at a domestic rate. There will be a fair use policy for data (internet access) but you will be informed if you exceed this. Many people turn mobile roaming data off but with the new EU rules, data charges are less of a concern.

If you want more data whilst abroad, why not add one of Asda Mobile’s Data Bundles. If you are travelling outside the EU you will be charged for using mobile data. Check out Asda Mobile’s roaming rates here.

How secure is my SIM card?

If you activate your SIM PIN then the SIM can’t be used in your phone or another phone without entering the code. SIM card cloning is possible but extremely rare. To clone a SIM, it needs to be removed from your phone and placed into a special cloning device.

Can I have two SIM cards with the same number?

No, you cannot have two SIM cards with the same number. There is one exception to this and it is called an eSIM. An eSIM is built into certain smartwatches so that people can leave their phone behind and still make and receive calls with the same phone number and under the same price guide.

Can my SIM create a personal hotspot?

Yes, you can create a mobile hotspot and connect your other devices to the internet. Not all networks allow you to do this but Asda Mobile do!

So there you have it… What else could you possibly want to know about a SIM card… Use this information wisely, with great power comes great responsibility.

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