What is the difference between IMEI and IMSI numbers?

IMEI and IMSI are two terminologies that are used frequently in the world of mobile communications. The number of connected mobile devices is growing at a rapid pace and the new 5G technology will only accelerate it. As more devices join the network, device identification becomes increasingly important in a world where device manufacturers are continuously evolving the identification methodologies. Accurate identification of mobile devices is vital for network operators for assigning the right services to the right devices. Mobile networks use various unique codes to be able to dispatch the services relevant to specific subscribers accurately. IMEI and IMSI are two such codes that are vital for the identification of mobile devices.

IMEI stands for International Mobile Equipment Identity and is a unique number assigned to every mobile device including mobile phones, dongles, data cards etc. IMSI stands for International Mobile Subscriber Identity and is a unique number assigned to the SIM card used by the mobile device.

The IMEI number is linked to the mobile device (e.g. your smartphone) while IMSI is linked to the mobile network (i.e. your SIM – Subscriber Identity Module). In case you are wondering, these concepts work the same way even if you are using a mobile phone with an eSIM. Let’s have a look at both of these unique codes in a bit more detail to find out what they do.

What is IMEI (International Mobile Equipment Identity)?

International Mobile Equipment Identity or IMEI is a unique number assigned to every mobile device including (but not limited to) mobile phones and mobile broadband data cards. It is a 15 digit number programmed into the mobile device by the manufacturers. This number stays with the mobile device over the entire lifetime of the device and must not be modified. Only the manufacturers of the mobile device are allowed to assign this number. The act of changing this number is called “unblocking” which is a criminal offence under UK laws.

In the earlier days and even now there are mobile phones that allow users to be able to remove the battery. This number can be found at the back of the phone behind the battery area. But the easiest way to find your IMEI number is to simply type *#06# on the dial pad of the phone, which will pop up a screen with the IMEI number written on it. In most smartphones from leading manufacturers, IMEI number can be found in the phone settings usually under “About” or “About device”.

IMEI number is unique to the mobile device, and it can be used to protect the phone from being misused if stolen. In case a mobile phone is lost or stolen, the customer should immediately contact their mobile service provider. If the phone is lost (and not stolen), the mobile service provider might only block the SIM, but if the phone is stolen, they can use the IMEI number to block the phone which will “blacklist” the phone from being used on any network.

Example of IMEI when you type *#06# on your cell phone

What is IMSI (International Mobile Subscriber Identity)?

International Mobile Subscriber Identity or IMSI is a unique number assigned to the SIM card of a mobile subscriber. IMSI is usually a 15 digit number that identifies the mobile user within the mobile network. In order to ensure confidentiality of the mobile user, the network uses a temporary number known as TMSI (Temporary Mobile Subscriber Identity) during most of the communication with the mobile phone.

IMSI is usually a 15 digit number where the first 3 digits represent the Mobile Country Code (e.g. 234 for the UK), the next 2 represent the Mobile Network Code (e.g. 15 for Vodafone UK), and the last 10 digits represent the Mobile Subscriber Identification Number. The Mobile Subscriber Identification number is the identity of the subscriber within the Public Land Mobile Network (PLMN). IMSI is used whenever the mobile phone tries to access the mobile network irrespective of which technology it is using i.e. 2G GSM, 3G UMTS, or 4G LTE.

As mentioned above also, the mobile network may use a temporary IMSI called TMSI (Temporary Mobile Subscriber Identity) instead of IMSI to ensure subscriber’s confidentiality. A user does not need to know or worry about their IMSI number, however, they can use available apps in app stores to be able to find out their IMSI number. Mobile operators use IMSI as the identity of the mobile subscriber to assign and manage any services allocated to that particular subscriber. As an example, the first two parts of the IMSI number i.e. Mobile Country Code and Mobile Network Code can indicate when a mobile user is outside of the country and roaming which can, in turn, allow mobile operators to apply the correct billing.

What about other codes like ICCID and MSISDN?

When we talk about IMSI and IMEI, we often come across some other inter-related numbers or codes that may be a bit confusing. The ‘usual suspects’ would be ICCID and MSISDN, so let’s clarify them:

  • ICCID: ICCID stands for Integrated Circuit Card Identifier and it is a number that identifies the chip of your SIM card (the shiny golden part of the SIM). In other words, it is the ID of your plastic SIM (or eSIM), irrespective of what your mobile phone number is. As you may know, you can get any SIM card (chip) from your mobile operator and get them to reassign the mobile number to that SIM. That’s why ICCID is important as it keeps track of the chip of your SIM or eSIM. ICCID number is stored in the SIM card but it is also printed on the SIM (long number with around 20 characters).
  • MSISDN: MSISDN stands for Mobile Station International Subscriber Directory Number and is actually a lot simpler than it sounds. It is your full mobile number including the country code and any prefixes (if applicable). For example, in the UK, the country code is +44 (or 0044) and the mobile numbers start with 07. When you write the MSISDN, you take away the + sign (or 00), and you take away ‘0’ from 07 to get something like this: 447xxxxxxxxx.
Example of ICCID and IMSI numbers

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