Without it, your phone is like Thor without his hammer – you might have the device in hand, but the power is missing. It's what opens the way for calls, texts, and the internet. What is a SIM card and why does so much depend on this unassuming piece of plastic? In this article, you'll find all the information worth knowing.
Key information from the article
- A SIM card is your network pass – it's responsible for user identification and enables calls, texts, and mobile internet.
- It contains key data – including ICCID, IMSI, phone number, and GSM network login.
- Security is provided by PIN and PUK codes, which protect network access.
- The evolution of SIM – from large cards in the 90s to nanoSIM and digital eSIM, which works without plastic and activates online.
- Dual SIM offers flexibility – two numbers in one phone, useful for travel or separating private from business life.
What is a SIM card and how does it work?
A SIM card (eng. Subscriber Identity Module) is a small chip with a processor and memory that allows your phone to connect to the mobile network operator's network. It is responsible for identifying the user in the system – thanks to it, you can make calls, send messages, and use mobile internet.
A SIM card stores, among other things:
- ICCID – unique serial number of the card (19–20 digits)
- IMSI – subscriber number in the network,
- Your phone number,
- login data for the GSM network,
- a small memory, which was previously used to store contacts and SMS messages, and today remains only as a trace of older technologies.
The connection to the network occurs via the contacts on the card, which communicate with the phone's modem. In practice, a SIM card therefore acts like a pass – the operator reads data from it and grants the device access to its network.
What security features does a SIM card have?
The SIM is a vault protected by two keys:
- PIN (Personal Identification Number) – a four-digit code, without which you won't get on board. You can change or disable it. (Here you can read more about how to disable your SIM card PIN).
- PUK (Personal Unblocking Key) – an eight-digit emergency code that will save you if you enter the PIN incorrectly three times.
Casting your net for more practical tips? Check out our guide where we advise on how to delete contacts from a SIM card.
How have SIM cards changed? A brief history
The first SIM card appeared in 1991 and was the size of a credit card. Back then, it took up so much space in the phone that little was left for anything else. Over the years, SIMs became smaller and smaller, while phones became larger and more advanced.
Why are SIMs shrinking? Because every millimeter in a phone is worth its weight in gold. A smaller slot = more space for the battery and components.
Today, we can use eSIM. What is it and what are its advantages?
eSIM (eng. Embedded SIM) is a card embedded in the device. Instead of a physical card, an eSIM profile is sufficient – you activate it online, scan a QR code, and you're on the network in a few moments. It's a practical solution: no need to wait for a courier, it's easy to change operators, and you can have several numbers on one device.
Dual SIM – two numbers in one phone
The Dual SIM function allows you to use two numbers on one device. It comes in handy when you want to separate your private life from your business life or use Polish and foreign cards simultaneously. In practice, this means one phone is enough for everything – you are simultaneously the captain and helmsman of two numbers.
We distinguish several Dual SIM variants:
- active – both cards operate simultaneously (two antennas in the phone),
- standby – only one card is active at a given moment, and the other "waits" until you finish your call,
- passive – you switch between cards manually.
Curiosities and practical tips
- Emergency calls without a SIM – even if the card goes overboard, you can call 112. The emergency number has the highest priority on the network and always works.
- SIMlock – in the past, operators would put a lock on phones so they only worked with their card. Today, it's just a relic that sank to the bottom.
- Card trimming – if you have an old SIM and a new phone, the operator will replace it with a newer format. And in service centers, you can also "trim" the card to micro or nano.
- Landline phone on SIM – yes, it exists! It looks like a classic landline but operates on a mobile network.
- Don't know why your phone isn't reading the SIM card? There could be several reasons – a dirty slot, a damaged card, or a minor system error. Most often, restarting the device, reinserting the card, or replacing it helps.
FAQ – frequently asked questions
1. Does a phone need a SIM card?
Yes, if you want to use calls, texts, and mobile internet. A digital eSIM is an alternative.
2. Can you use a phone without a SIM card?
Yes – you can use it via Wi-Fi, as a camera, a gaming console, or a music player. But without a SIM (or eSIM), you cannot make calls or send texts.
