IMSI

The IMSI (International Mobile Subscriber Identity) is the number a mobile network uses to identify a subscriber. It is stored in the SIM or eSIM profile, sent during network registration, and used by the home network to find the subscription and authenticate the device. Its format is defined in ITU-T Recommendation E.212 and 3GPP TS 23.003.

IMSI structure

An IMSI is at most 15 digits long:

Part	Length	Meaning
MCC	3 digits	Mobile country code — the home network’s country
MNC	2 or 3 digits	Mobile network code — the home operator within that country
MSIN	Up to 10 digits	Mobile subscription identification number, assigned by the operator

The MCC and MNC together form the home network’s PLMN identity (see MCC/MNC). An IMSI beginning 310 260 belongs to a US network with a three-digit MNC; one beginning 234 15 belongs to a UK network with a two-digit MNC.

How the IMSI is used

• Registration: on first attach, or when the network has no temporary identity for the device, the device sends its IMSI. The visited or serving network uses it to route authentication to the home subscriber database.
• Privacy: after the first attach, networks assign temporary identities (such as the GUTI in LTE and 5G) to limit how often the IMSI goes over the air. In 5G, the IMSI-based permanent identity (SUPI) is encrypted into a SUCI before it is sent.
• Roaming: the visited network reads the MCC/MNC to identify the home network and check for a roaming agreement.
• Multi-IMSI profiles: some IoT and travel profiles carry several IMSIs and switch between them, so the device presents a different home network depending on location.

IMSI vs. MSISDN and ICCID

The IMSI is not the phone number. The MSISDN is the dialable number linked to the IMSI in the operator’s systems, and the ICCID is the serial number of the SIM or profile. Data-only IoT subscriptions often have no MSISDN, but nearly every cellular subscription has an IMSI.

Why it matters for testing

The IMSI decides which networks will accept a device. If the visited network has no agreement with the IMSI’s home network, or the subscription is barred there, registration fails with a reject cause such as #11 “PLMN not allowed” (see attach reject cause). SimCheck.ai tests real profiles on real modems and shows the decoded signaling — including the network’s reject cause — for each attempt. See roaming testing.