What is an IMSI and how is it related to the phone number?

In the world of mobile telecommunications, the IMSI (International Mobile Subscriber Identity) plays a vital role in identifying and authenticating users on cellular networks. Though often confused with a phone number, the IMSI serves a very different — yet fundamentally important — purpose. Understanding what an IMSI is and how it relates to a phone number helps clarify how mobile networks function behind the scenes.

1. What Is an IMSI?

The IMSI (International Mobile Subscriber Identity) is a globally unique identifier assigned to every mobile subscriber in a cellular network. It is stored on the SIM card and is used by the mobile network to recognize and authenticate the subscriber.

The IMSI is never directly visible to end users.

2. Structure of an IMSI

An IMSI is usually a 15-digit number, though it can be shorter in some cases. It consists of three main parts:

1. MCC (Mobile Country Code) – 3 digits

   • Identifies the country of the subscriber’s home network.

   • Example: 310 for the USA, 404 for India.

2. MNC (Mobile Network Code) – 2 or 3 digits

   • Identifies the mobile recent mobile phone number data network operator (carrier).

   • Example: 260 for T-Mobile USA.

3. MSIN (Mobile Subscriber Identification Number) – Remaining digits

   • A unique number identifying the subscriber within the network.

Example IMSI: 310260123456789

• MCC = 310 (USA)

• MNC = 260 (T-Mobile)

• MSIN = 123456789

This IMSI identifies a unique subscriber on T-Mobile’s network in the USA.

3. Where Is the IMSI Stored?

The IMSI is embedded in the significance of phone number data in marketing the SIM card (Subscriber Identity Module) and read by the mobile device when it connects to the network. When a mobile phone is powered on, the device uses the IMSI to initiate communication with the mobile network.

4. How the IMSI Is Used by Mobile Networks

When a mobile device tries to access a network, it sends the IMSI to the operator’s core network. The network verifies this identifier using its internal authentication systems (typically via an HLR — Home Location Register or modern equivalents like HSS in 4G/5G).

When you travel abroad, the foreign network identifies your home carrier through your IMSI and communicates with your home operator to authenticate and enable roaming services.

Operators use IMSI to track email data usage (calls, data, SMS) and apply correct charges, especially in postpaid billing systems.

D. Lawful Interception and Security

IMSI helps governments and carriers track usage, intercept calls (with legal authority), and trace fraud or illegal activities.

5. IMSI vs. Phone Number (MSISDN)

Though often linked, an IMSI is not the same as your phone number. Here’s how they differ:

6. How IMSI and Phone Numbers Interact

• When someone calls your number, the network looks up the MSISDN and maps it to the associated IMSI.