Multiple International Mobile Subscriber Identities stands for multi-IMSI which is a relatively new way of storing multiple IMSIs on a SIM card. Using multiple IMSIs, a device can connect to more than 1 carrier if required.
An IMSI consists of three distinct digit parts:
MCC which stands for Mobile Country Code. This is the first part which stands for the country of operation.
MNC or Mobile Network Code. It shows a corresponding MNO.
Mobile Subscription Identification Number. This part is unique for every subscriber.
MNOs leverage the IMSI information to determine the geo (country) and Mobile Network Operator associated with a particular subscriber.
Usually, one IMSI is associated with a definite region (country) and MNO. However, when a device goes beyond its home network, it establishes connections with other MNOs listed as network roaming partners. Although primary MNOs in their home countries seldom offer redundant coverage, multi-IMSI SIM cards ensure it by dealing with lists of approved networks. In cases where an IMSI lacks available networks, the multi-IMSI system changes for another IMSI automatically with relevant network options. The IMSI swapping process involves several steps, including provisioning the SIM with the active IMSI, location updates, and the applet's automatic switch to the new IMSI based on the device's location.
Although both Multi-IMSI and eUICC technologies facilitate carrier switching and storage of several MNO profiles, they have fundamental differences. Multi-IMSI SIM enables simultaneous storage of multiple carrier subscriptions, eliminating the need for frequent switching. At the stage of SIM Card manufacturing specific profiles are carefully selected and preloaded onto the card. Subsequent modifications or removals of profiles are not possible. While all profiles remain active, the SIM card can be programmed to follow predetermined rules to determine which profile to prioritize. For example, if a device transitions between countries where it possesses network subscriptions, it can be programmed to switch to the corresponding profile automatically. In case of eUICC-enabled SIM cards, additional carrier profiles can be conveniently downloaded onto eUICC-capable SIMs OTA based on the user's specific requirements. Once a Multi-IMSI SIM has been inserted into a device, the user's control diminishes as the rules and carrier profiles are already programmed within the SIM card's system prior to deployment.
