What an IMEI check tells you before you buy a used phone — and what it does not

Every phone sold through a legitimate channel carries an IMEI: a serial number tied to the radio hardware rather than to the SIM, the account or the owner. It is the one identifier that survives a factory reset, which is why every guide to buying second-hand starts with it. It is also the identifier most often oversold. An IMEI check answers a narrow set of questions very well and says nothing at all about the rest, and the difference matters when you are standing in front of a seller with cash in your hand.

Where the number lives, and why you should read it in three places

Dial *#06# and the phone prints its IMEI from the radio firmware. A dual-SIM handset prints two, one per radio, usually differing in the last digits. The same number is printed on the SIM tray or the back panel on many models, engraved into the chassis on some, and stamped on the retail box. It also appears under the settings menu, filed with the model and serial number.

Read it in at least three of those places and compare. A mismatch between the dialled number and the box means the box is not this phone's box — common enough with resold stock, and by itself not proof of anything worse. A mismatch between the dialled number and the engraved or printed one on the body is a different matter: the chassis has been swapped, or the number has been altered. Either way the warranty trail is broken, and in most jurisdictions altering the identifier is an offence in its own right.

The check digit

An IMEI is fifteen digits, and the last one is a checksum over the first fourteen computed with the Luhn algorithm — the same scheme used on payment cards. A number invented on the spot almost never satisfies it. That makes the checksum a useful five-second filter against a number typed into a listing at random, and useless against a number copied from a real phone.

What the checks answer

There are three separate questions people collapse into one, and they are answered by three different systems.

• Is the handset blocked? Operators report lost and stolen devices to a shared blocklist, and a blocked IMEI is refused service on the networks that honour it. This is the check worth running first, because a blocked phone is a paperweight with a camera.
• Is it the model it claims to be? The leading digits of an IMEI encode the type allocation of the model. A lookup will return a make and a model name; if that does not match the phone in your hand, stop.
• Is it under warranty? Warranty status lives with the manufacturer, keyed to the serial number and the date of first sale. Several makers publish a coverage lookup. This is also the check that quietly reveals a grey import: a handset built for another market often returns no local coverage at all.

What no IMEI check will tell you

It will not tell you whether the battery is tired, whether the screen is an original panel or a replacement, whether the phone has been opened, or whether it has been under water. It will not tell you whether the seller owns it. It will not tell you whether the bootloader has been unlocked — see our note on what unlocking does to a warranty — and it will not tell you whether the software has been replaced. Those questions need the phone in your hand, powered on, with the settings menu open.

Nor is a clean result permanent. A blocklist entry is added when the loss is reported, which can be days or weeks after the sale. A phone bought on a financing plan that is later defaulted on can be blocked long after money has changed hands. A clean check on the day tells you the phone is not blocked on the day.

The order to run them in

Do the cheap checks before you travel to meet anyone. Ask the seller for the IMEI in the listing, run the checksum and the model lookup, and confirm the model matches the photographs. If the number is refused, the conversation is over at no cost.

On meeting, dial *#06# yourself rather than reading a screenshot, compare it against the tray or the chassis, and open the settings page that shows model, serial and build. Then run the block check while you are standing there, on your own connection. Only after that is it worth looking at the screen, the charging port and the battery — the parts of the inspection that take real time.

A note on paperwork

Ask for the original invoice. It carries the IMEI, the date of sale and the seller of record, and it is what a manufacturer's service centre will ask for if a covered fault appears later. A phone with a matching invoice is worth paying more for than the same phone without one, because the invoice is the only document that links the handset to a lawful chain of sale.

If you are weighing a used unit against a refurbished one from a seller with a stated return window, the labels matter more than they look — we set out what each one covers in refurbished, open box and seconds. And before you commit to any handset that came from another market, check that its radio bands match the networks you actually use: the method is in reading a band list.