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User Guide: What is Stored on a SIM Card


Wondering what is stored on a SIM card? Don’t worry, you’re not alone. Most people like you are wondering what makes this small chip so powerful that millions of people are today able to talk from one end of the world to another.

A Subscriber Identity Module (SIM) card is a tiny electronic chip that is inserted into a mobile phone to authenticate and identify the user’s mobile network. The SIM card serves as a key to the mobile network and contains a range of important information.

Firstly, it stores the user’s unique International Mobile Subscriber Identity (IMSI) number, which is essential for connecting to the network. The IMSI number helps the network identify the user and provides access to the mobile network. The SIM card also stores the mobile number assigned to the user by the network operator. This number is used to make and receive calls, send and receive text messages, and access mobile data.

Additionally, the SIM card can also store phonebook contacts, text messages, and other data such as settings, service provider information, and security keys. Moreover, modern SIM cards also support advanced technologies such as Near Field Communication (NFC) and Global Positioning System (GPS). With NFC technology, users can make contactless payments, access public transport services, and exchange data between devices. On the other hand, the GPS feature allows location-based services such as emergency calls, location tracking, and navigation.

In summary, a SIM card is an essential component of any mobile phone as it stores critical information that enables access to mobile networks and advanced features.

Types of a SIM Card

Before diving in deep, let’s look at different SIM cards. Basically, a UICC (Universally Integrated Circuit Card) SIM is categorized into four types: 1FF, 2FF, 3FF, and 4FF. However, the microchip that stores the information is the same size in all the categories. Other than a UICC SIM card, there’s another type called e-SIM, which comes in previously mentioned types.

Standard SIM – 1FF

1FF was probably the first size of UICC SIMs introduced in the market. 1FF SIM measures equal to the size of a debit card. The dimensions of a 1FF UICC are 85 × 54 × 0.76 mm.

However, this size suddenly became obsolete after the Mini-SIM arrived. Today 1FF SIM is no longer in use for modern electronic gadgets.

Mini-SIM – 2FF

Even though the name says “Mini,” it’s likely the giant SIM available nowadays. It was the smallest SIM size when it was first introduced back in 1995.

Measuring 15 x 25mm, the Mini-SIM was once used in many smartphones, including Apple’s iPhone 3GS. Some basic or old mobile phones still work on the Mini-SIM, but modern smartphones don’t. Today the Mini-SIM has been widely replaced by Micro and Nano SIMs.

Micro SIM – 3FF

The Micro SIM is about half the size of a Mini-SIM, which means it can be inserted into devices with much less space available for the slot.

SIM works on smartphones and tablets but not laptops or desktop computers. It was first introduced in 2003 and was widely used in most smartphones until 2015. In later years, the manufacturers shifted to the newer Nano-SIM.

Nano SIM – 4FF

First introduced in 2012, Nano-SIM is the tiniest and youngest of the group. Measuring only 8.8 x 12.3mm, the Nano SIM has no border surrounding it. It’s challenging to imagine SIM cards growing much smaller in the future until the size of the microchip is altered.

Nano SIMs are by far the most common size available. So, if you’re purchasing a new phone, you’ll probably require one.


An eSIM or embedded SIM lets you connect to any network offering eSIM plans. You can switch to your favourite network without buying a separate SIM. An eSIM works the same as a standard SIM without needing a physical SIM card. It is already installed in smartphones at the time of manufacturing.

Besides smartphones, eSIM is also used in smartwatches, drones, location trackers, and vending machines. A fully equipped eSIM allows you to select a second plan if you’re travelling or you just need a second number for work.

To activate the e-services, you only need to install your favourite carrier’s “eSIM profile”. You can switch between two carriers easily with just a tap. The latest smartphones by Apple, Samsung, and Google allow users to use an eSIM and a regular SIM card.

How does a SIM Card Work on Smartphones?

A SIM card contains a gold-plated microchip made of silicon and other metals. This microchip contains security circuits and a processor which acts as a controller. When you insert the SIM into your phone, the operating system starts reading it. Ultimately, the phone transfers all the SMS and phone numbers to your SIM card.

SIM cards connect a specific number to a particular mobile phone. It helps network companies identify which number is linked to which customer. All the communication we do is encrypted in a secret code, and the key to that secret code is stored in the SIM card.

Network companies allocate a unique IMSI (International Mobile Subscriber Identity) number to every SIM card. When someone dials your number, the network identifies that a call is being made to your IMSI number. Hence the network directs that call to your phone instead of someone else’s.

What Information is Stored in a SIM Card?

The data stored in a SIM card jacket includes user identification, location, phone number, and network authorization information. Moreover, the other information including personal security keys, contact lists, and saved text messages are just a few of the data SIM cards can hold.

  • Advice of Charge (AoC): The AoC is a part of the GSM service that determines the cost of mobile services activated on the SIM.
  • Authentication Key (Ki): A unique key allocated differently to every SIM card. Its size is 128-bit and used to validate SIM cards on a certain network provider.
  • International Mobile Subscriber Identity (IMSI): A unique number that links your SIM card to your network provider.
  • Country Code (MCC): The country code is the first three digits of your IMSI number. Combined with the Network Code, the country code identifies GSM networks worldwide.
  • Mobile Subscriber Identification Number (MSIN): This number works as a link between a particular user and the network provider.
  • Mobile Station Integrated Services Digital Network (MSISDN): The phone number provided to the user for making calls and texts. Your MSISDN could be changed if requested.
  • Local Area Identity (LAI): Works as an identifier and includes Location Area Code (LAC), network code, and country code.

How does the SIM Storage Work When Travelling Abroad?

If you take your SIM along when traveling abroad, your phone will automatically turn on the roaming mode. International Roaming connects your SIM to a specific local network operating in your travel destination.

Network carriers usually keep a set of up to 80 preferred networks. This selection is stored in the form of numbers on every SIM. When the network identifies any SIM going outside of the preferred network selection, for example, abroad, the SIM will automatically connect to an international carrier that’s best in range.

What is the Cost of a Standard SIM Card?

Buying a SIM card in the UK is more convenient and cheaper than in any other country. If you walk down any busy street in the UK, you’ll find a lot of dedicated shops and convenience stores that sell SIM cards. The major network providers – O2, EE, Vodafone, and Three – do not have much difference in cost and monthly plans. However, several MVNOs (Mobile Virtual Network Operators) offer much better rates than the major brands.

MVNOs are basically the resellers that use the coverage from previously mentioned major brands but design their own monthly plans and contracts. Hence, they offer their own PAYG SIM cards at much cheaper rates.

Talk Home, one of the cheapest MVNOs in the UK, offers free PAYG SIM without any contract. You can also order your free SIM through Talk Home’s website, and it will be delivered to your doorstep without any additional charges.

If we talk about the coverage, Talk Home uses the EE network – one of the oldest and largest network providers in the UK. So, you can expect great network availability even in the rural parts of the country.

Moreover, Talk Home offers its monthly plans for as low as £5 a month. If you’re a new customer, you’ll get 50%

Apple initially introduced the SIM lock. It’s a function in GSM smartphones that runs off on all monthly plans for the first three months. You can explore their cheap monthly plans here.

Talk Home is also one of the most trusted networks among ex-pats. Their international plans offer the cheapest calling rates worldwide. So, you can connect with loved ones anywhere in the world for as low as 1 penny/cent per minute. Check Talk Home’s international calling rates here.

What is a SIM lock?

Apple initially introduced the SIM lock. It’s a function in GSM smartphones that temporarily restricts phone usage over a certain network provider. The primary reason behind introducing a SIM lock was to ensure that subscribers complete the tenure of their service contract.

For example, if a customer has agreed to a 12-month serviced contract with EE, they can’t use any SIM other than the EE for up to 12 months. After the contract period is completed, the phone can be easily unlocked with a code provided by the carrier, and the customer is free to use any of their desired networks.

How to Remove SIM Lock from an iPhone and Android?

You can only remove the SIM lock from an iPhone if your contract period from the first network is over. If all the unlocking requirements are met, you can submit the “unlock request” to your network provider. Your carrier will check the contract status before unlocking your iPhone.

After your phone is unlocked, you can remove the old SIM and insert the new one; it should work without interruption. If you see a message that says, “The SIM card does not appear to be supported,” follow these steps.

  • Open settings, go to General, and tap on “About.”
  • A message should appear under the Carrier Unlock option that says, “No SIM restrictions.”
  • If you don’t find this message, contact your network provider, and ask whether there’s something else that needs to be done to unlock your SIM.

For android phones, the process is pretty much the same. If your contract period is over, contact your carrier and ask them to unlock your SIM. Your carrier will probably ask for the IMEI number of your phone. You can check the IMEI number by dialing *#06#.

After getting the IMEI number, your network provider will remove the SIM lock within 2-3 days.

Pros and Cons of a SIM Card


  • You don’t need to change the number every time you change your phone. You can use the same number for your lifetime.
  • You can save up to 250 contacts in your SIM memory. That way, you can’t lose your important contacts if you lose your phone or forget your iCloud or Gmail password.
  • A SIM card reader can be connected to your PC and allows you to download your contact information directly into your computer.


  • Having a SIM card means you can only use one number at a time. You won’t be able to use your phone’s multi-line feature.
  • Sim cards are prone to wear and tear over time
  • A SIM card can easily be taken out of your phone, so there’s always a risk of getting your contacts stolen. However, activating a PIN lock on your SIM can prevent such things.

Final words

What is stored on a SIM card isn’t as complex as others think it is. There is no doubt that this small chip is one of the most marvelous things made by humans. If you’re living in the UK, make sure to get your hands on a free SIM card from Talk Home Mobile and enjoy the widest network coverage across the UK.


1. What is a SIM card?

A SIM card is a tiny electronic chip that is inserted into a mobile phone to authenticate and identify the user’s mobile network.

2. What is stored on a SIM card?

A SIM card stores the user’s unique International Mobile Subscriber Identity (IMSI) number, mobile number, phonebook contacts, text messages, and other data such as settings, service provider information, and security keys. Modern SIM cards also support advanced technologies such as Near Field Communication (NFC) and Global Positioning System (GPS).

3. Why is the IMSI number important?

The IMSI number helps the network identify the user and provides access to the mobile network.

4. Can I transfer my SIM card to another phone?

Yes, you can transfer your SIM card to another phone as long as it is compatible with the network and the phone is unlocked.

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Comment (1)
  1. Techylist 1 year ago

    I’m glad to see that Talk Home stores a lot of information on a SIM card. This will make it easier for people to keep track of their account and phone numbers.


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