IMSI
IMSI is the unique identifier for a SIM card on a cellular network — not to be confused with IMEI (which identifies the phone itself). 15-digit number split into MCC (country), MNC (operator), and MSIN (subscriber).
IMSI (International Mobile Subscriber Identity) is a 15-digit unique identifier for a cellular subscriber. It lives on the SIM card (or eSIM profile) and identifies you to the carrier’s network. It is distinct from IMEI, which identifies the phone hardware.
Structure
An IMSI breaks into three parts:
- MCC (Mobile Country Code): 3 digits identifying the country. US carriers all use 310, 311, 312, 313, 314, 315, 316.
- MNC (Mobile Network Code): 2-3 digits identifying the carrier within that country. Verizon = 004, 010, 012; T-Mobile = 020, 026, 160; AT&T = 030, 410, 950; Dish = 410.
- MSIN (Mobile Subscriber Identification Number): the remaining digits identifying you uniquely.
IMSI vs IMEI vs phone number
Three different identifiers users sometimes confuse:
- IMSI (on the SIM): identifies the subscriber to the network.
- IMEI (in the phone): identifies the hardware. Carriers use IMEI to allow/block stolen phones.
- Phone number (E.164 format): the externally-routable address others dial. Mapped to your IMSI by the carrier’s HLR.
When you port your number to a new carrier, the phone number stays; the IMSI changes (you get a new SIM/eSIM with a new IMSI). When you swap phones but keep the SIM, the IMSI stays; the IMEI changes.
Privacy
The IMSI is sensitive because it can be used to track you on cellular networks (so-called "IMSI catchers" or "Stingrays" in law-enforcement context). Modern 5G adds privacy protections (concealed-identifier signaling) that reduce the IMSI’s exposure on the air interface.