IMSI

IMSI is a globally unique identifier that mobile networks use to authenticate and route communications for a specific subscriber. It is stored securely on the SIM or eSIM and transmitted to the network during registration. Unlike a phone number, the IMSI is a low-level network identifier invisible to the end user and rarely changes.

How it works

The IMSI is a 15-digit string composed of the Mobile Country Code (MCC, 3 digits), Mobile Network Code (MNC, 2 or 3 digits), and Mobile Subscriber Identification Number (MSIN). When a device powers on and registers with a cell tower, it transmits the IMSI so the Home Location Register (HLR) or Home Subscriber Server (HSS) can authenticate the subscriber and retrieve their service profile. To protect against IMSI catchers (stingrays), modern LTE and 5G networks increasingly use a SUCI (Subscription Concealed Identifier) that encrypts the IMSI before transmission.

Key facts

• Structure: MCC (3 digits) + MNC (2-3 digits) + MSIN (up to 10 digits) = max 15 digits total
• Privacy concern: IMSI catchers (fake cell towers) exploit IMSI transmission to track devices; 5G’s SUCI mitigates this
• Per-profile on eSIM: Each eSIM carrier profile contains its own IMSI, which is why switching profiles changes your network identity

For builders

Builders rarely need to interact with the IMSI directly, but understanding it clarifies why switching an eSIM profile effectively makes the device look like a completely different subscriber to the network. This is relevant when testing dual-SIM setups or diagnosing roaming issues where the home network fails to authorize a foreign network attachment.

Sources

• GSMA. eSIM specifications (SGP.21, SGP.22) for consumer Remote SIM Provisioning. gsma.com
• GSMA. The State of the eSIM Market whitepaper. gsma.com
• ITU. Recommendation E.118: International telecommunication charge card / ICCID structure. itu.int
• 3GPP. Mobile network specifications portal. 3gpp.org
• FCC. Consumer guides on wireless devices and unlocking. fcc.gov