Just under a year ago, the US arm of telecomms giant T-Mobile admitted to a data breach after personal information about its customers was offered for sale on an underground forum.

At the time, VICE Magazine claimed to have communicated with the hacker behind the breach via online chat, and to have been offered “T-Mobile USA. Full customer info.”

VICE’s Motherboard reporters wrote at the time that:

The data include[d] social security numbers, phone numbers, names, physical addresses, unique IMEI numbers, and driver licenses information, the seller said. Motherboard has seen samples of the data, and confirmed they contained accurate information on T-Mobile customers.

IMEI is short for International Mobile Equipment Identity, a globally unique serial number burned into your phone when it’s manufactured. Because the IMEI is considered a “non-resettable identifier”, apps on both Android and iOS are restricted from accessing it unless they have been granted special device management privileges, and developers are instructed to rely on user-resettable identifiers such as advertising IDs when legitimately tracking users and devices. You can view your phone’s IMEI by dialling the special phone number *#06#.