IMSI numbers are usually 15 digits, and they have three distinct parts:

1. The first set of digits is the Mobile Country Code (MCC), which defines the country a subscriber primarily operates within. This is always either two or three digits.
2. The second set of digits is the Mobile Network Code (MNC), which identifies the specific MNO a subscriber is associated with. This is between one and three digits.
3. The final set of digits is the Mobile Subscription Identification Number, which is unique to the subscriber. (This is typically nine or ten digits.)

 

For example, here’s what you can learn from the IMSI number 310410123456789:

 

Mobile Country Code	310	United States
Mobile Network Code	410	AT&T
Mobile Subscription Identification Number	123456789

 

An IMSI is not the same as an Integrated Circuit Card Identification (ICCID) number. While they are both parts of the SIM profile, an IMSI identifies the subscriber, whereas the ICCID identifies the SIM card itself. Multiple SIM cards will have the same IMSI if they’re associated with the same subscriber, but they will each have a unique ICCID.

Every cellular-enabled device has an IMSI number stored within its SIM card, and when the device attempts to connect to a mobile network, the MNO uses the IMSI to authenticate it.

While every IMSI comes from a specific Mobile Network Operator, they allow a device to attach to networks from more than one MNO. Which networks your device can connect to depends on the agreements your MNO has negotiated with other providers.

Any time a device attaches to a network that doesn’t belong to the subscriber’s primary MNO, the device is considered “roaming,” and the subscriber will pay adjusted rates for any services they use on the network.

Your IMSI has an associated network coverage list that includes your MNO and any Network Roaming Partners they have agreements with.