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IMEI, IMSI, MSISDN, and TAC: cellular identifiers without the confusion
A practical guide to the identifiers for devices, subscriptions, phone numbers, and device models.
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Mobile networks need to distinguish a device, a subscription, and a dialable phone number. Those are different things, which is why cellular data contains several identifiers that look similar but should not be used interchangeably.
| Identifier | Identifies | Typical structure | Useful for |
|---|---|---|---|
| IMEI | Mobile equipment | 15 digits | Equipment checks and device history |
| TAC | Equipment model allocation | First 8 digits of an IMEI | Model-family lookup |
| IMSI | Mobile subscription on a SIM or eSIM | MCC + MNC + MSIN, up to 15 digits | Authentication and subscriber records |
| MSISDN | Dialable telephone number | E.164 number, up to 15 digits | Calling, messaging, and customer-facing services |
flowchart TD Device[Device or radio] -->|has| IMEI IMEI -->|starts with| TAC[Type Allocation Code] SIM[SIM or eSIM subscription] -->|has| IMSI IMSI <-->|mapped by the operator| MSISDN[Phone number] Device <-->|currently uses| SIM
The International Mobile Equipment Identity is assigned to mobile equipment. A 15-digit IMEI consists of an eight-digit Type Allocation Code, a six-digit serial number, and a check digit. Multi-SIM phones commonly expose more than one IMEI, so “one handset equals one IMEI” is not a safe assumption.
Networks can compare an IMEI with an Equipment Identity Register (EIR). This supports allow, watch, or deny decisions for equipment independently of the SIM currently in it. It is useful for stolen-device programs and some fraud controls, but it is not a perfect human identifier: devices can be repaired, identifiers can be missing or duplicated, and malicious equipment may spoof values.
In this context, TAC means Type Allocation Code, the first eight IMEI digits. It can map an observation to allocation data such as manufacturer and model family.
There is an easy telecom naming trap: in LTE and 5G mobility discussions, TAC also means Tracking Area Code. The surrounding field—IMEI versus location or tracking area—tells you which one is intended.
The International Mobile Subscriber Identity belongs to the subscription stored on a SIM or eSIM profile. Its structure is:
IMSI = MCC + MNC + MSIN
An IMSI is sensitive network data. Modern systems try not to expose it repeatedly over the air, using temporary identities instead. In 5G terminology, an IMSI can form the identifier part of a SUPI, while SUCI protects it during initial access.
The MSISDN is the public number used to route calls and messages. The operator keeps a mapping between subscription records and one or more MSISDNs, but the number is not the subscription itself. A number can be changed or ported, and some data-only subscriptions may not have a normal dialable number at all.
This distinction matters in analytics. Counting MSISDNs measures numbers; counting IMSIs measures subscriptions; counting IMEIs approximates equipment. None of those automatically equals a unique person.
Suppose a geofenced service applies a network policy when a subscription enters a restricted area. A control keyed only by IMSI follows the SIM, so swapping SIMs can make several short-lived subscriptions appear on the same equipment. Correlating IMSI and IMEI observations can reveal that pattern, while TAC adds model context.
That does not mean every SIM change is fraud. Dual-SIM use, device resale, repairs, shared phones, roaming, and incomplete observations all create legitimate many-to-many relationships. A production rule should use time windows and corroborating signals, retain the minimum data needed, and support review rather than treating one identifier as proof.
The normative structures live in 3GPP TS 23.003. GSMA manages the official IMEI and TAC allocation process.
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