What is a SIM Card and What is it Used For?

Many people often carry SIM cards in their phones without knowing what actually it is. All they will tell you is without a SIM card, my phone won’t work or I cannot make a call.

So they start to wonder when they are faced with challenges such as SIM swapping, because honestly that has become a common threat in our society today.

Therefore we don’t want to leave you blue all the way not knowing what your SIM card is and what it is actually used for.

Let’s get started!

A SIM card stores your information as a subscriber. In other words, it’s an integrated circuit designed to store the subscriber’s identity number and used to identify and authenticate the subscribers on specific mobile devices.

The kind of information the SIM card holds include:

  • The subscriber contacts
  • User data
  • SMS messages
  • Dialed calls
  • Network data
  • The Serial Number of the SIM known as Integrated Circuit Card Identifier(ICCID)
  • The phone number assigned to the SIM card abbreviated as MSISDN and the Identifier that ties the SIM to a specific user account known as the International Mobile Subscriber Identity(IMSI)

Since the SIM stores your information, it won’t matter whether you are using Android or Apple. When your phone runs out of battery, you can take hold of your SIM card and make a call or text from a phone with a battery.

And if you just bought a new phone, you don’t have to switch your SIM card rather you can simply transfer your SIM into the new phone provided it uses the same SIM card size.

Therefore if you just acquired a new smartphone and wonder whether your phone can actually work without a SIM card: – The answer is “YES

Your Android or iPhone Smartphone will work very well without ever needing a SIM card. All you will need is a WIFI (internet Connection) and a couple of Apps to use on the device.

The only limitation is when you want to make calls or receive SMS/MMS messages. You will need an active cell service to send and receive the SMS or MMS messages to and fro.

Phones like iPhone can be used without a SIM Card but the tricky part is activating and setting the phone in the first place.

And the only way to activate the cell service is when you insert a SIM card for GSM cellular carriers.

Depending on your geographical location, SIM cards are purchased at different prices. The only similarity is that they all carry Activation fees and SIM card costs

To see what’s on your SIM card,

=>Go to “Settings” on your phone

=>Tap “About Phone

=> Select “SIM Status” to see your Phone number, service status and roaming info.       

Sim Card Parts and Functions

The Sim Card has 6 parts with each carrying different functionality. The most important of all is the microcontroller which is a paper sized chip. Its typical ROM has a size between 64KB to 512 KB. The ROM size ranges between 1KB to 8KB.

The EEPROM contains data called personalization that includes security keys, MS settings, phonebook among others.

The operating voltage of the SIM is estimated around 1.8V, 3V, or 5V. However the operating voltages of most of the modern SIM support over 5V, 3V, and 1.8V.

1. Integrated Circuit Card Identifier(ICCID)

This is the primary Account Number and it is a 19 digits string. It contains sections like Issuer Identification Number (IIN), Individual Account Identification, and Check digit among others.

2. International Mobile Subscriber Identity(IMSI)

It is used to identify the carrier service and the individual is using it. It’s also a 19 digits string. The first 3 digits represent the Mobile Country Code (MCC), the next 2 to 3 digits represent the Mobile Subscriber Identification Number (MSIN).

3. Authentication Key(Ki)

It is 128 bit used to authenticate the SIM card on the Mobile Network. The Authentication Key is unique for every user as assigned by the operator. The authentication key is also stored in the database of the carrier’s Network.

When the mobile phone first activates using the SIM card, it gets the International Mobile Subscriber Identity (IMSI) from the SIM card and transfers it to the mobile operator for authentication.

The database queries incoming International Mobile Subscriber Identity (IMSI) and its associated Authentication key.

The operator database then generates a Random Number (RAND) and signs it with the IMSI and gives another number called the Signed Response 1(SRES_1).

The Random Number will be sent to the mobile phone and the SIM then signs it with the Authentication Key and produces the SRES_2 which then passes into the operator network. The operator network comprises the SRES_1 it produced and the SRES_2 from the mobile phone. If the two correlate, the SIM is authenticated.

4. Location Area Identity(LAI)

It is the data stored in the SIM Card about the local Network available in that particular area. The operator network is divided into different small areas each having a location Area Identity.

5. SMS Messages

The SIM card can store many SMS

6. Contacts

It can also store around 250 Contacts.               

Frequently Asked Questions

Does a SIM card hold pictures?

The answer is “NO”. The sim card simply stores your personal information and in case you need to save photos, and videos, your android or iPhone has an SD card slot or you can choose to save your photos directly to your phone provided it has enough internal storage space.

The Camera functionality will allow you to take photos instantly.

Is SIM card necessary to transfer data between two phones?

You don’t necessarily have to use a SIM Card to transfer data because the data can be stored in the phone’s internal storage rather than the SIM card. However, some phones may require a SIM card installed to utilize the data on the phone.

Do you need to remove your SIM card before factory reset?     

No, you shouldn’t. Unless when you’re performing a factory reset and select “erase SIM card memory” then it will delete all the memory storage on the SIM.

Factory reset brings the phone to a state when it was still brand new and therefore only erases the smartphone data. Nothing else.


Knowing your sim card functionality today is as important as knowing how to use your phone. Dive into our comments section and share with us how much you know about your sim card.

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Written by Mila Joshua Yona

Pro South Sudanese Blogger, Digital Marketer & Web Designer. I help entrepreneurs scale up their businesses online. You can join my Facebook Group Here or Telegram Group Here

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