What is IMSI (International Mobile Subscriber Identity)?

International Mobile Subscriber Identity, IMSI, is a unique number automatically generated and stored in the SIM. It identifies every mobile phone subscriber on a UMTS or GSM network.

The number is stored on the SIM card and is not moved or changed when that mobile number (MSISDN) is ported to a different SIM card. It is, therefore, a fundamental way of preventing SIM swap fraud.

How is the International Mobile Subscriber Identity code generated?

Each IMSI number has three parts. The initial part is the mobile country code (MCC), followed by the mobile network code (MNC), and the third part is the mobile subscription identification number (MSIN).

Mobile country code – The MCC defines the country a subscriber primarily operates within—usually two or three digits.

Mobile network code – The MNC identifies the specific MNO a subscriber is associated with—between one and three digits.

Mobile subscription identification number – The network operator allocates the MSIN to identify the subscriber uniquely—usually, nine or ten digits.

For example, 310 260 123123123

310 – MCC  (for USA)
260 – MNC (for T-Mobile)
123123123 – MSIN (unique to you)

What is IMSI analysis?

When a mobile phone connects with the provider’s network, the phone sends the code to the network.

IMSI analysis is the automatic process of analyzing a subscriber’s code to identify their home network and determine whether subscribers from that network are allowed access (if they are not local subscribers, then a roaming agreement must be in place).

The IMSI is an essential part of the Home Location Register (HLR) lookup process as it is used to identify which network a mobile number belongs to. The process is sometimes called IMSI Lookup or Number Lookup.

What is the role of International Mobile Subscriber Identity in international roaming?

Suppose a connecting mobile belongs to a subscriber from another network provider. In that case, the IMSI is automatically converted into a Global Title, which can be used to look up the subscriber’s data in their own operator’s HLR. This process facilitates international mobile roaming.

Once the IMSI has been converted, SCCP routes the message to its destination.

The challenges with IMSI numbers for IoT

1. Roaming

IMSI SIM is usually tied to a specific Mobile Network Operator (MNO) and country. IoT devices might lose connectivity when crossing borders or need local SIM cards, increasing complexity and cost.

Negotiating roaming agreements between operators for each country where your IoT devices operate can be expensive and time-consuming.

2. Flexibility

With traditional IMSIs, you’re often locked into a specific MNO, limiting your ability to negotiate pricing or switch providers if service quality is poor in a particular location.

Managing IMSI

Traditional SIM cards have limited flexibility, requiring a physical swap to change connectivity providers.  Modern multi-IMSI SIMs and eSIMs revolutionize this process, enabling over-the-air (OTA) IMSI management.


Storing multiple IMSI profiles on a single SIM provides fallback options in case of network failures or roaming restrictions.

Automatic switching between networks ensures your IoT devices remain connected to the best available network.


Embedded SIMs allow for remote provisioning and profile updates over the air.

This facilitates switching mobile network operators seamlessly, providing your devices with the best possible service, data rates, and uptime.

How does IMSI help prevent fraud?

SIM swapping is a technique fraudsters use to take over a person’s mobile identity to steal cash and assets.

SIM swap checks can be utilized to prevent this activity automatically. The silent process looks up the IMSI number connected to the SIM card to check if it has recently been changed. For example, a financial services provider using an OTP might use a SIM swap check to ensure they aren’t sending the PIN to a fraudster using a new SIM card with the same mobile number as an account they are trying to hack.

What is the difference between IMSI, ICCID, and IMEI numbers?

IMSI, ICCID, and IMEI are all essential for proper cellular network functioning and play a role in security measures to prevent unauthorized access. But,

ICCID number is an 18-20 digit code uniquely identifying each physical SIM card. It’s often printed on the SIM card itself.

IMEI is a 15-digit code uniquely identifying a mobile phone or other cellular device. It’s like the device’s fingerprint.

The IMSI and ICCID work together on the SIM card to identify the subscriber and the specific SIM card.

The IMEI identifies the device itself, allowing the network to verify that it’s a legitimate device, not a stolen one.

IdentifierWhat it identifiesPurpose
IMSIThe mobile subscriberAuthentication, billing, service provision
ICCIDThe physical SIM cardSIM identification, linking to IMSI, security
IMEIThe mobile device itselfDevice identification, theft prevention, compatibility


Mar 18th, 2024
4 min read

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