Quiz Entry - updated: 2026.07.14
What is the IMSI, how is it structured, and how does it differ from the phone number?
The IMSI (International Mobile Subscriber Identity) is a globally unique 15-digit number stored on the SIM (UICC), composed of MCC + MNC + MSIN — distinct from the subscriber's dialable phone number (MSISDN).
Structure (15 digits):
| Part | Meaning |
|---|---|
| MCC | Mobile Country Code (e.g., 228 = Switzerland) |
| MNC | Mobile Network Code (operator, e.g., 03 = Salt) |
| MSIN | Mobile Subscriber Identification Number (the individual user) |
IMSI vs. MSISDN — a crucial distinction:
- IMSI = who the SIM is (the identity used to log into the network); stored on the UICC; not normally exposed to other users
- MSISDN = the phone number people dial to reach you
These are deliberately separate. Your phone number can change (port it to a new SIM) while the IMSI stays bound to a specific SIM, and vice versa. The network maps one to the other in the HLR.
Why the IMSI is sensitive: it's the permanent handle for a subscriber. Exposing it enables tracking — which is the whole reason for the TMSI.
Go deeper:
International mobile subscriber identity (Wikipedia) — the MCC + MNC + MSIN breakdown and how IMSI relates to MSISDN.