What is IMSI (International Mobile Subscriber Identity)?

An IMSI is a unique and automatically generated code stored in the SIM that identifies every mobile phone subscriber on any network globally.

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

Each IMSI number has two parts. The initial part is comprised of six digits in the North American standard and five digits in the European standard. It identifies the subscriber’s GSM network operator in a specific country. The second part is allocated by the network operator to uniquely identify the subscriber.

When a mobile phone connects with provider’s network, the code is sent by the phone 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 important 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.

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

Once the IMSI has been converted, SCCP is used to route the message to its final destination.

SIM swapping is a technique used by fraudsters to take over a person’s mobile identity with the intention of stealing cash and assets.

SIM swap checks can be utilized to automatically prevent this activity. 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 make sure 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.