- What a SIM actually proves
- How your number relates to the SIM
- What still works without a SIM
- How to read SIM locks and bands
The short answer
A SIM card is a small chip that tells the mobile network, "this phone may use this person's contract." Your phone number is not stored in the handset itself; the carrier keeps the link between your SIM, your number and your plan on its own systems. That is why a phone with no SIM can still run apps and browse the web over Wi-Fi. What stops working is everything that uses the mobile network: calls on your number, SMS and mobile data.
The SIM is a membership card for the network
Think of a gym membership card. Anyone can see the building from the street, but only card holders get to use the equipment. A phone is the same. The carrier's signal may be all around you, but unless the SIM proves there is a valid contract, the network will not let the phone in.
In slightly more precise terms, a SIM (Subscriber Identity Module) holds a number that identifies the subscription and secret information used to authenticate it, so that nobody else can pose as you. When the phone powers on and connects to a base station (the antenna equipment that carries mobile signals), this information is used to check that the SIM belongs to a genuine contract. The secret part is designed never to leave the chip, and users are not expected to alter what is on it.
It helps to separate the two ideas: the SIM represents the contract, and the handset is the machine that does the talking. The handset has its own identifier, too, called the IMEI, which is unique to each device.
Your number and your contract
You will often hear that "your number is on the SIM." More accurately, the carrier manages the number. Inside the carrier's systems, this SIM, this phone number and this price plan are tied together. The SIM is the key that calls up that record.
A few practical consequences follow from this.
- Move the SIM to another phone and you can make calls from that phone on the same number, provided the handset supports the network.
- If you lose the SIM, contact the carrier. They will block it and issue a new one. Because the number belongs to the contract, it carries over to the replacement. Procedures and any fees vary, so check your carrier's guidance.
- Switching carriers means swapping to the new carrier's SIM. Taking your number with you is called number portability (MNP in Japan).
The name on the account, your plan, and how much data you have used this month all live on the carrier's side as well. You cannot see your remaining data by looking at the SIM. For that, open your carrier's app or account page.
Physical SIM and eSIM
Everything so far describes the card you slide into a tray on the side of the phone, known as a physical SIM. An eSIM is a chip built into the phone at the factory, onto which the contract details are written over the network. The job is identical. The only difference is whether the contract arrives as a card or as data.
| Point | Physical SIM | eSIM |
|---|---|---|
| Form | A card in a tray | A chip built into the phone |
| How you get it | By post or over the counter | Written to the phone over the network |
| Using it in another phone | Move the card across | A transfer or reinstall step is needed |
| Phones that support it | Any phone with a SIM tray | Phones with eSIM support |
In Japan, the Ministry of Internal Affairs and Communications published guidelines to promote eSIM in August 2021 and asked the major carriers to offer it. An eSIM can sometimes be up and running without waiting for a card to arrive, but changing phones then involves steps on screen rather than a simple swap. The next article covers this in detail.
What works with the SIM removed
Here is what a phone can do with no SIM, or with a SIM from a contract that has ended.
| Feature | No SIM, on Wi-Fi | No SIM, no Wi-Fi |
|---|---|---|
| Calls on your phone number | No | No |
| SMS to your number | No | No |
| Web, apps, video | Yes | No |
| Free calls in messaging apps | Yes | No |
| Camera, saved photos and music | Yes | Yes |
An old phone left over after an upgrade can still play music around the house or serve as a homework device for a child, all over Wi-Fi. The catch is SMS verification. If your online bank or another service sends a login code by text message, a phone without a SIM will never receive it.
How emergency calls behave without a SIM varies by device and situation, so it is not covered here.
Before sending a phone for repair, giving it away or trading it in, take the SIM out. Leave it in, and someone else can make calls and receive texts on your number.
SIM sizes and SIM locks
Sizes and capabilities
Physical SIMs come in several sizes. Recent phones commonly use the small size known as nano-SIM, while some older handsets take a larger one. Japan's ministry also notes that SIM cards differ in size and capability, so you should choose one that matches your device. If a sign-up form asks you to pick a size, check your phone's specifications first.
SIM locks
A SIM lock restricts a phone bought from one carrier so that it only works with that carrier's SIMs. According to the ministry, SIM locks are in principle prohibited on phones released in Japan from October 2021 onward. Phones bought from the major carriers before then may still be locked. Unlocking is free when done online; over the phone or in a shop, a fee may apply.
An unlocked phone will not necessarily work equally well on every network. If the frequency bands the phone supports do not match those the network uses, reception can be patchy in some places. When you insert another carrier's SIM, you may also need to enter APN settings (the details that tell the phone where to connect for data) before data or VoLTE (the system that carries voice calls over the 4G network) will work. If the new carrier publishes a list of devices it has tested, that list is a good guide.
In summary
A SIM is a membership card for a carrier's network. Your number and plan are managed by the carrier, and the SIM is the key that calls up that contract. Without a SIM, Wi-Fi still gets you online, but calls on your number and SMS do not work.
Checking four things about your own phone now will save a scramble when you switch carriers or upgrade.
- Whether it uses a physical SIM, an eSIM, or can take both (look under mobile or cellular in Settings)
- Whether it is SIM-locked
- Which services send you login codes by SMS
- If you plan to use another carrier's SIM, which frequency bands it supports and how to set the APN