Mobile Fundamentals · SIMs & Identity · Lesson 1 of 4

The Three Numbers

IMEI, IMSI and ICCID, and which one belongs to what.

Ask the phone who it is

Every phone that connects to a mobile network has a serial number called the IMEI, short for International Mobile Equipment Identity. It belongs to the hardware. Swap the SIM, reset the phone or change carriers, and the IMEI stays the same. Phones with two SIM slots usually have two IMEIs, one for each slot.

An IMEI is 15 digits long. The first 8 digits are the Type Allocation Code, which identifies the make and model. The next 6 are that unit's own serial number. The last digit is a check digit, calculated from the other 14 with the Luhn formula, the same one used on credit card numbers. A single mistyped digit makes the check fail.

You'll need your IMEI to unlock a phone, report it stolen or make an insurance claim. Carriers in many countries can block a stolen phone by its IMEI, and investigators use it to link a physical handset to network records.

The quickest way to see it is a short code built into nearly every phone. Dial *#06# on the simulated phone and count the digits.

Device vs subscriber

Three different numbers come into play when your phone uses a mobile network. Each one identifies something different.

• IMEI identifies the phone hardware. It's built into the device.
• IMSI (International Mobile Subscriber Identity) identifies you as a customer of a carrier. It's stored on the SIM and is up to 15 digits long, starting with a 3-digit country code and a 2- or 3-digit network code.
• ICCID is the serial number of the SIM itself. It's usually 19 or 20 digits, starts with 89, and is often printed on the card.

Your phone number is none of these. The carrier links your number to your IMSI in its own systems, which is why moving a SIM to another phone takes your number with it.

This split matters in practice. Put your SIM in a friend's phone and the network sees your IMSI paired with their IMEI. Carriers can log those pairings, so investigators can sometimes trace which handsets a SIM has been used in. The IMSI is also what fake cell towers called IMSI catchers try to collect. 5G networks can encrypt it to make that harder.