What is an IMEI, and what does blacklisting a phone do?
An IMEI is a unique identifier for a mobile device — the International Mobile Equipment Identity — and blacklisting it prevents that specific handset connecting to mobile networks, regardless of which SIM is inserted.
What it identifies. The device, not the subscriber. The SIM identifies the subscriber; the IMEI identifies the hardware. A phone keeps its IMEI when the SIM changes, which is what makes blacklisting effective.
How to find it: dial *#06#, check the settings under device information, look on the SIM tray or the original box, and — for dual-SIM phones — expect two IMEIs.
What blacklisting does. When a handset is reported lost or stolen, the IMEI is added to a shared register — in the UK, a central equipment identity register — and participating networks refuse service to it. The phone still works over Wi-Fi and as a device; it cannot make calls or use mobile data on those networks.
Its limitations, which matter:
Coverage is not universal. Networks in some countries do not participate, which is why stolen handsets are exported and sold in markets where blacklisting is not enforced.
IMEI reprogramming exists, and is illegal in the UK under specific legislation, but it happens.
Blacklisting is distinct from other locks. A phone can be blacklisted, carrier-locked, and subject to an activation lock simultaneously — and the three are removed by different parties. Manufacturer activation locks tied to an account are generally the more effective deterrent, because they make the handset unusable rather than merely unable to connect.
Why it matters when buying second-hand. Always check the IMEI before buying. A phone blacklisted or reported as still on finance may look and behave perfectly at the point of sale and become useless afterwards. Free and paid checking services exist, and this single check prevents the most common second-hand purchase failure.
Reporting promptly matters — report to the network and the police, and obtain a crime reference number, which insurers require.
Blacklisting can be reversed if a phone is recovered, by the network that reported it.