Fundamentals BroadNet answer

What is an IMSI?

Last reviewed July 22, 20261 min readReviewed by BroadNet engineering

Quick answer

IMSI (International Mobile Subscriber Identity) is the globally unique identifier a mobile network uses to recognise a subscriber. Stored on the SIM/eUICC and in the HLR/HSS, it is up to 15 digits: a Mobile Country Code (MCC), a Mobile Network Code (MNC), and a subscriber identity (MSIN). Unlike the MSISDN — the dialable phone number — the IMSI lives on the signalling plane and is never exposed to other subscribers.

Plain-language definitionVendor-neutralReviewed July 22, 2026

The long answer

Everything worth knowing, nothing that isn't.

§01

IMSI vs MSISDN

The MSISDN is the external routing number; the IMSI is the internal network identity. The two are decoupled: MNP lets a subscriber keep their MSISDN while their IMSI stays with the SIM. An SS7 SendRoutingInfoForSM query returns the recipient's IMSI plus the serving MSC — which is exactly why that operation is a fraud target. See /answers/what-is-msisdn and /answers/ss7-map-send-routing-info-for-sm.

§02

Why the IMSI is sensitive

Because the IMSI ties directly to a real subscriber, its exposure enables tracking (the classic 'IMSI-catcher' attack) and SMS interception. 5G conceals the permanent identity behind the SUPI/SUCI scheme so it is never sent in the clear. Operator signalling firewalls guard IMSI-revealing queries at the network edge. See /operators/ss7-firewall.

BNWritten by the BroadNet engineering team·Operating carrier-grade messaging since 2003, headquartered in Dubai·Editorial standards

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