What Is a SIM Card?
A SIM card (Subscriber Identity Module) is the small chip that sits inside your smartphone. Your phone number and contract details are recorded on it, and its job is to tell your carrier's network something along the lines of "this handset belongs to so-and-so, and their phone number is such and such".
Without a SIM card you cannot make calls or use mobile data (4G/5G). You can still connect to Wi-Fi, but calls and internet access while you are out of the house are gone.
The easiest way to think of a SIM card is as your phone's identity document. Just as you cannot withdraw money at a bank without proof of identity, your phone cannot make calls or reach the internet without a SIM card. What the card holds includes your phone number, information about the carrier you have a contract with, and encryption keys (the keys that keep your communications secure).
SIM Card Sizes
SIM cards come in three sizes.
- Standard SIM: the largest (25mm × 15mm). Used in older mobile phones
- Micro SIM: the middle size (15mm × 12mm). Used in earlier smartphones
- Nano SIM: the smallest (12.3mm × 8.8mm). The size most widely used in smartphones
Nano SIM is the mainstream size for smartphones. If the size does not match when you change handsets, a mobile phone shop will swap the card for you. SIM cards have grown smaller year by year, and the reason is to make better use of the space inside the phone.
What Is an eSIM?
Some smartphones support what is called an eSIM (embedded SIM). Rather than a physical card, an eSIM writes your contract details onto a chip built into the phone itself.
- No swapping cards. You can sign up and activate online
- You can use two numbers on one handset (dual SIM). That lets you keep work and private life apart
- When you travel abroad, you simply add a local eSIM and you are on the local network
- There is no risk of losing it, as there is with a physical SIM
Whether a handset supports eSIM depends on the model. You can check in your phone's settings screen or on the manufacturer's official website.
The "dual SIM" arrangement, which pairs an eSIM with a physical SIM, is also handy for anyone who wants to run a low-cost SIM alongside a major carrier. You might, for instance, keep your data costs down on the low-cost SIM while making calls on a major carrier's unlimited calling plan.
What Happens if You Take the SIM Card Out?
- You can no longer make or receive calls
- You can no longer send or receive SMS
- Mobile data (4G/5G) stops working
- Wi-Fi still works
- Offline features such as the camera, music and games still work
In other words, a smartphone with the SIM card removed is in much the same state as a small Wi-Fi-only tablet. Understanding the difference between LINE calls and ordinary phone calls shows you how to keep talking over Wi-Fi even with no SIM. You can also carry on using an old handset as a Wi-Fi-only music player or camera.
Handling a SIM Card Safely
- A SIM card is a precision component. Do not bend it or let it get wet
- Do not lend it to anyone (calls and SMS would then be made on your phone number)
- If you lose it, contact your carrier at once and have the line suspended
- Setting a PIN code on the SIM card prevents misuse even if it is stolen
- Do not touch the metal contacts on the card. Fingerprints and dirt cause poor connections
Losing a SIM card, or having one stolen, is a serious problem because it can lead to your phone number being used fraudulently. If you drop your phone or it is stolen, contact your carrier straight away and ask them to suspend the SIM card. Carriers provide a contact point for loss and theft. Check the opening hours and the number for the carrier you are with in advance, on its official website.
SIM Locks and SIM-Free Handsets
In the past, smartphones sold by carriers carried what was known as a SIM lock. A SIM lock is a restriction that allows only one particular carrier's SIM card to be used. A phone bought from Docomo, for example, would take only a Docomo SIM.
Under a Ministry of Internal Affairs and Communications guideline that took effect on 1 October 2021, setting a SIM lock without a legitimate reason is not accepted as a general rule. Smartphones sold from that point onwards therefore do not carry a SIM lock as a general rule (they are SIM-free). Because any carrier's SIM card will work in them, you no longer need to buy a new handset when you switch carriers with MNP. If a handset sold before then does carry a SIM lock, you can ask the carrier to remove it. Since 1 October 2023, handsets on which a SIM lock was set in the past are unlocked free of charge on request.
What Is a Low-Cost SIM?
A low-cost SIM is a SIM card you can use for less than the major carriers (Docomo, au, SoftBank) charge. Companies known as MVNOs (mobile virtual network operators) provide the service by renting network capacity from the major carriers. Plans generally cost less than the major carriers charge, so they can cut your communications spending.
The drawbacks of a low-cost SIM are that the connection can slow down at busy times such as the lunch break and the early evening, and that with some services there are few physical shops, so support is mostly online. Considering it together with ways to save on call charges lets you optimise your communications spending as a whole. If speed matters a great deal to you, take a look at the major carriers' online-only plans as well (ahamo, povo, LINEMO).
What Are APN Settings?
When you start using a low-cost SIM, you may have to do a piece of setting up called the APN configuration. The APN (Access Point Name) is the configuration information your phone uses to connect to the internet. A major carrier's SIM configures itself automatically, whereas a low-cost SIM sometimes has to be set up by hand. Each low-cost SIM provider explains how to do it in detail on its official website.
Summary
A SIM card is your phone's identity document. It holds your phone number and contract details, and without it you have neither calls nor internet. Do not lend it to anyone, contact your carrier immediately if you lose it, and set a PIN code. Those are the three things to remember. The eSIM, which does away with the physical card altogether, is also widely available, widening the range of ways you can use a phone.