IMSI — International Mobile Subscriber Identity
What is IMSI?
An IMSI (International Mobile Subscriber Identity) is the unique number stored on a SIM that identifies a specific mobile subscriber to mobile networks worldwide, distinct from the phone number itself. It's built from an MCC and MNC identifying the subscriber's home network, followed by a mobile subscription identification number (MSIN) unique to that subscriber. Networks use the IMSI internally for authentication, roaming, and HLR/HSS lookups — it typically never appears to the end user.
More detail
The IMSI is fixed to the SIM, not the phone number: it stays the same when a subscriber gets a new handset but changes if they switch SIMs, which is why porting a number moves the phone number to a new IMSI on the new carrier's network rather than moving the IMSI itself.
A number lookup or HLR lookup resolves a phone number to its current IMSI-linked network behind the scenes, then reads off that same record's MCC/MNC to determine the subscriber's home and current serving network for routing and rating.
Frequently asked
- Is IMSI the same thing as a phone number?
- No. A phone number is what callers dial and is portable between carriers; the IMSI is a separate, carrier-assigned identifier stored on the SIM that networks use internally for authentication and routing, and it changes when the subscriber's SIM changes even if the phone number stays the same.
- Why does a messaging or voice platform care about IMSI?
- Platforms don't usually handle raw IMSIs directly, but the same lookups that resolve a number's current MCC/MNC and carrier for routing and pricing rely on IMSI-linked network records, so IMSI is the underlying identifier that makes accurate carrier detection possible.
See also
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