What is IMSI Number (International Mobile Subscriber Identity)?

Mark Kreitzman
calender icon
October 5, 2022
Modified On
May 17, 2024

What is IMSI?

The International Mobile Subscriber Identity, or IMSI, is a unique number assigned to every mobile phone used by cellular networks to identify valid devices and allow them to be connected to the network. The number consists of up to 15 digits. The IMSI identifies a specific GSM subscriber, in other words. 

There are two elements in this range. The first has six digits and is intended for usage in North America, while the second has five digits and is used in Europe. This number identifies which GSM network provider provides service to a specific country where the subscriber holds an account. The network operator assigns this second portion to identify said subscribers uniquely from others.

The Subscriber Identity Module (SIM) in the phone receives the IMSI from the handset, which is then transmitted to the correct network. The IMSI is used to look up mobile information in the Home Location Register or Visitor Location Register.

When a mobile is linked, it receives a temporary IMSI and is used to identify the user in subsequent exchanges. This is built into the SIM of the mobile device and transmitted whenever the network is accessed. It's sent during startup initialization.

To maintain the subscriber's confidentiality and anonymity, their IMSI is only transmitted rarely. Instead, a temporary mobile subscriber identity (TMSI) that has been generated randomly is sent. This eliminates the possibility of eavesdroppers catching and deciphering the IMSI over radio links.

SIM Swap Protection

Get our SAFE plan for guaranteed SIM swap protection.

Protect Your Phone Now

How Does The IMSI Number Work?

The IMSI (or international mobile subscriber identity) is a 64-bit integer found on SIM cards. It works by linking to a mobile network, which then has the ability to connect to other networks. The R-UIM card can be used to provide up to three phones at once. The R-UIM card has no limit on the number of phones it may provide; however, there are restrictions on the network types that may be supported by the phone. Numerous networks, including GSM, CDMA, and EVDO, are possible. The phone or the R-UIM card is immediately activated when provisioned in a phone (for example, through an SMS message).

IMSI stands for International Mobile Subscriber Identity and is made up of three components: the MCC, MNC, and MSIN. The MCC (Mobile Country Code) is a three-digit code that designates which country the mobile subscriber is located in. The MNC (Mobile Network Code) identifies which mobile network operator they're using and  is two or three digits long. Lastly, the MSIN (Mobile Subscriber Identification Number) is ten digits long and uniquely identifies each individual mobile subscriber within their network provider. The government assigns both function similar to location area identifier—the main difference being that IMSI covers a broader range international locations.

When you turn on your phone, it searches for the network and sends out its IMSI (International Mobile Subscriber Identity) number. The IMSI attach procedure happens when a phone first connects to any available network. If the phone changes locations, it notifies the VLR of updated location information using an updated VLR (Visitor Location Register). This process occurs until the subscriber's HLR (Home Location Register), which contains all their subscriber info, is reached. The position of the mobile station is updated regularly. The phone is not deactivated if the updating time expires but has not been registered. If the mobile station is unregistered after the deadline, it will not be deactivated. The IMSI detaches when a phone is shut off to notify the network that it is disconnected.

IMSI Number Structure

An IMSI number is comprised of 15 digits with three distinct parts:


MCC stands for the Mobile Country Code is the first set of the digits (usually 2 or 3 digits) that defines the subscriber's country.


MNC stands for the Mobile Network Code is the second set of digits (usually 1-3 digits) that identifies the subscriber's MNO (Mobile Network Operator).


MSIN stands for the Mobile Subscription Identification Number is the final set of digits (usually 9-10 digits) is unique to each subscriber to identify them in the database.

Global IMSI Number

If you're looking for an IMSI that will work in various countries and with various carriers, you'll need to look into a global IMSI. This can be obtained from an MVNO that uses the home network of a large carrier. You can also acquire this by signing up for a prepaid SIM card that supports multiple countries and has a global IMSI.

There are a few things to keep in mind when you're looking for a global IMSI:

  • Check the list of countries where your IMSI will work.
  • Check what data rates are available in each country—some may be slower than others.
  • Be aware of any restrictions or special requirements that come with each country.

Remember that a global IMSI will not work with every carrier in every country, but it's a good way to have more flexibility when connecting your devices to cellular networks. When you're looking for an IMSI for your IoT devices, make sure to keep these things in mind. A global IMSI may be your best option, depending on your needs.

Also Read: What is Multi IMSI Number?

SIM Swap Protection

Get our SAFE plan for guaranteed SIM swap protection.

Protect Your Phone Now

IMSI Number FAQs

When Is The IMSI Number Used?

Users who do not need to know or be concerned about their IMSI number can find this number using programs available in app stores. Mobile carriers use the IMSI to identify and manage any services assigned to the subscriber. The first two components of an IMSI number may show whether a mobile user is roaming outside of their nation, allowing mobile carriers to charge the correct amount.

How to Track An IMSI Number

Only the phone subscriber or those working for the operator or police enforcement with a legal right (such as a warrant) may access information, and they must know the phone's most recent location.

The network knows which phone is on, when it's on a call, and how much data it's using. A cellular provider knows the location of a smaller set of cells than the cell tower does.

If a phone is turned off, networks can only know which call the phone was on last if the phone remains switched off.

What's The Difference Between An IMSI And An ICCID?

Even though they are both components of the SIM profile, an IMSI and an ICCID number aren't the same. The IMSI identifies the subscriber, while the ICCID pinpoints the SIM card. If many SIM cards are linked to one customer, they will have identical IMSI numbers; nevertheless, each has a unique ICCID number.

All cellular-enabled devices have an IMSI number stored in the device's SIM card. This allows mobile network providers to verify a customer when they attempt to connect to their network.

Your IMSI defines which carrier network your phone is locked to. However, depending on your carrier's agreements with other providers, you may be able to use your device on more than one network.

A device is considered "roaming" when it connects to any network that isn't the subscriber's primary service provider. Subscribers will then have to pay additional charges for any services they use while on a roaming network.

The IMSI has a network coverage list that encompasses the MNO and any Network Roaming Partners they have agreements with. If your phone connects to a signal outside your MNO's network coverage area, it will automatically search for the strongest signal from a Network Roaming Partner. Your phone will automatically reconnect to your MNO's network once you return to an area with their network coverage.

SIM cards connected to the same line of service might share the same IMSI but will each have their own globally unique ICCID, even if they're linked to the same SIM card.

Why Is The IMSI Critical For Cellular IoT?

The IMSI number is very important in device authentication. For example, if an IoT device tries to connect to a network, the IMSI will be part of the identification process that grants it access. Both the SIM and IMSI have a secret key called Ki. They work together to create authentication for devices trying to access mobile networks.

In addition to authentication, the IMSI is also used for network roaming. The IMSI is linked to your home network and assigned by your MNO. When a device leaves the area—for example, an asset tracker on a cross-country journey—other networks can identify it by its IMSI and determine agreements and rates for roaming.

MNOs must be able to send IMSIs and Kis OTA to a device's SIM with the introduction of eUICC-enabled eSIMs. Endeavour's security features are built into its eSIMs to protect these critical and unique identifiers.‍

What Are The IMSI Problems with IoT?

The IMSI and Ki offer an authentication solution that works well for consumer devices, which don't regularly need to use more than one SIM card or line of service. However, in cellular IoT deployments, the requirements are unique. For example, if a micro-mobility company wanted to deploy its connected scooters in Australia and across the U.S., it would have difficulty staying connected if it only installed a single IMSI. That's because mobile network operators (MNOs) are regional—meaning the devices could only connect within the MNO's home network region. Devices that only utilize a single-IMSI SIM will have difficulty connecting outside their specified area, or they will need to sign a roaming agreement. Right now, there are several limitations with cellular IoT devices and single-IMSI SIMs.

The Limitations of a Single Network

By having only one IMSI associated with a device, you limit that device to one carrier. This makes the device solely dependent on that carrier's infrastructure, resulting in outages and downtime for your IoT devices—and negative impacts on your business model. In addition, if the rates go up for that specific carrier, you are forced to pay those higher prices though you're receiving no additional benefits.

Inadequate Coverage

Your SIM must connect to a single IMSI only to the network defined in your mobile network code (MNC) and mobile country code (MCC). If you're using a regional carrier, your gadget may work fine inside your home, but if it's connected to a car or sent to another location, it might have trouble keeping up. It will be able to connect only to carriers on your MNO's Roaming Partners list, which might not always be the greatest choice in an area.

What Is An IMSI Checker?

The IMSI Checker is a program that allows you to check the IMSI of your mobile phone. The IMSI number can be very useful in various situations. For example, you may want to ensure that a SIM card is active or find out if someone has stolen your phone by checking for changes to the IMSI number.

An IMSI checker needs the IMSI number to work. After putting the number in, it will give you its status. This could be if it is active or any other details about it.

The IMSI checker is a great tool to verify the status of an IMSI number quickly. This can come in handy if you're trying to determine if a SIM card is active or not or if you think a phone may have been stolen and want to confirm by checking the IMSI number.

Another benefit of utilizing an IMSI checker is that it may provide more information about the number. If you put an IMSI number into a checker tool and the tool says the number is no longer operational, for example, you may be able to figure out when it was last active. This data can come in handy in various scenarios, such as trying to locate the owner of a lost or stolen phone.

Several IMSI checker tools are available online if you are searching for one. Some of these are free applications, while others might need a subscription or a one-time payment. Before making a decision, think about your requirements and the features that are most important to you. You may begin using an IMSI checker tool to verify the status of IMSIs after you've chosen one.

The IMSI checker tools may be useful in a variety of situations. They can help you verify the status of an IMSI number and determine whether a phone has been stolen and the IMSI number has been changed. It's vital to think about your requirements and the features that are most important to you before making a decision. After you've made your choice, you can start using it to look up the status of IMSI numbers.

What Is The Role of IMSI in Modern Technology?

IMSI is important in mobile technology because it helps identify network subscribers. Additionally, IMSI numbers are used in subscriber authentication, which is a process that helps to ensure that only authorized users can access a network. Moreover, IMSI numbers can be used to determine whether a mobile user is roaming and, as such, help mobile providers apply the proper invoicing. Finally, IMSI numbers can be used to trace a mobile device. However, this is typically only possible if the user is a phone subscriber or works for the operator or police enforcement with a legal right (such as a warrant) and knows the most recent location of the device.

Is IMSI Same as ICCID?

No, IMSI number is not the same as an ICCID number. Even though IMSI and ICCID are both components of the SIM profile, an IMSI number recognizes the subscriber, whereas the ICCID number recognizes the SIM card itself. It means multiple SIM cards associated with the same subscriber will have the same IMSI number but different ICCID numbers.


Now that you know all about IMSI numbers, you might wonder what's next in mobile technology. 5G is the next generation of mobile technology currently being developed. 5G promises to offer faster speeds and more reliable connections than previous generations of mobile technology. Additionally, 5G is expected to enable a new class of applications and services that are impossible with current technology.

Also Read: What Is An IMSI Catcher?

Mark Kreitzman
General Manager, Efani Secure Mobile

I am General Manager at Efani Secure Mobile. I have over 20 years of experience in enterprise cybersecurity, including 8 years focused on mobile security. I have a high level of achievements and expertise in the Service Provider Alliance/Partner space. I am positioned at senior levels with SPs/Resellers such as AT&T, Verizon, Cisco Systems, and many others. I have deep experience in SaaS, cybersecurity, telecom, mobile & blockchain.

Related Articles

SIM SWAP Protection

Get our SAFE plan for guaranteed SIM swap protection.