IMEI

The IMEI (International Mobile Equipment Identity) is a 15-digit number that uniquely identifies a mobile device to cellular networks, independent of the SIM card or eSIM profile inserted in it. Within SIM & eSIM technology it is the hardware half of identification: the SIM identifies the subscriber, while the IMEI identifies the phone, tablet, modem or IoT module that is using the network.

Networks use the IMEI to recognize device models, block stolen handsets and apply equipment-specific behavior, and manufacturers use it for warranty and repair tracking. Because it is tied to the hardware and not to a contract, the IMEI stays the same when the user changes operator, number or SIM.

Structure of the IMEI number

An IMEI has a fixed structure defined by 3GPP specifications and administered through allocations from the GSMA. The first eight digits identify the device model, the next six identify the individual unit, and the last digit is a checksum that detects typing errors.

A closely related format is the IMEISV, which has 16 digits: the TAC and serial number followed by a two-digit software version number instead of the check digit. Networks often request the IMEISV so they can tell which firmware version a device runs. In 5G systems the device identifier is formally called the Permanent Equipment Identifier (PEI), which for 3GPP devices is normally the IMEI or IMEISV.

The Luhn check digit catches every single-digit error and most swaps of adjacent digits, which is why mistyped IMEIs are usually rejected immediately by registration and warranty systems. It does not prove that a number is genuine, because anyone can compute a valid check digit.

How networks use the IMEI

When a device attaches to a network, the core can send an identity request and receive the IMEI or IMEISV, typically over a protected signaling connection. The operator can then query an Equipment Identity Register (EIR), a database that classifies devices as allowed, blocked or under observation.

Operators share lists of lost and stolen devices through a central device registry maintained by the GSMA, so a handset blocked by one operator can also be refused by others that participate. The TAC also lets networks apply device-specific settings, such as enabling voice over LTE only for models certified to support it, or identifying IoT modules for special tariffs.

• Theft deterrence: blocked IMEIs cannot register on participating networks, which lowers the resale value of stolen phones.
• Lawful interception and investigation: authorities can request records linked to a device rather than to a SIM.
• Network analytics: aggregated TAC data shows which device models and capabilities are present on the network.
• Customer support: operators use the IMEI to check compatibility, unlock status and warranty coverage.

IMEI compared with other mobile identifiers

The IMEI is often confused with the identifiers stored on the SIM or eSIM. Each identifier answers a different question, and mixing them up leads to wrong assumptions about what a network or an operator can track or block.

A Dual SIM phone usually has two IMEIs, one per SIM slot or logical line, because each line registers on its network as a separate piece of equipment. An eSIM-capable phone additionally has an EID, which identifies the embedded chip used for remote SIM provisioning and should not be confused with either IMEI.

Finding and checking an IMEI

Users can display the IMEI on almost any phone by dialing `*#06#`, which is a standardized code. It is also shown in the device settings, printed on the retail box, and on many devices printed on the SIM tray or the back cover.

Before buying a used phone, comparing the IMEI shown by `*#06#` with the box and checking it against a blocklist lookup offered by an operator or registry helps avoid a device that will not register on the network.

Limitations, risks and misconceptions

The IMEI is a strong identifier, but it is not a security credential. It is transmitted to networks, shown on screen and printed on packaging, so knowing a device’s IMEI does not grant access to it or to the user’s account.

• IMEI changing is illegal in many jurisdictions, because altered numbers defeat theft blocking and investigation.
• Cloned IMEIs exist on counterfeit devices, so a valid-looking number does not guarantee an authentic phone.
• Blocking is not universal: a device blocked in one country may still work on networks that do not use the shared registry.
• Sharing the IMEI publicly has privacy implications, because it is a stable identifier that links activity to one device over time.
• A factory reset does not change the IMEI, contrary to a common belief among sellers of used devices.

The idea in brief

Every phone has a serial number that the mobile network can read, and that number is the IMEI. Your SIM card says who you are as a customer, while the IMEI says which physical phone you are using.

It works a little like the chassis number of a car. You can change the license plate, the owner or the insurance, but the chassis number stays with the car. That is why a stolen phone can be blocked by its IMEI even after the thief swaps the SIM card.

A practical scenario

A commuter loses a phone on a train. At home, they find the original box and photograph the label showing two IMEI numbers, because the phone supports Dual SIM. They call their operator, which suspends the SIM to prevent misuse and adds both IMEIs to the shared blocklist of lost and stolen devices.

Two weeks later, someone inserts a new SIM into the phone. The network checks the device identity during registration, finds both IMEIs on the blocklist and refuses service. The phone can still use Wi-Fi, but it can no longer make calls or use mobile data on participating networks, which makes it much less valuable to resell.

The outcome shows why keeping a record of the IMEI matters: the blocking was possible only because the owner could supply the exact numbers quickly.