IMEI (International Mobile Equipment Identity)
Updated
The IMEI is a 15-digit number that identifies a mobile device, more precisely its cellular modem, independently of the SIM inside it. It contains a type allocation code that identifies the model, a serial number and a check digit. Its format is defined in 3GPP TS 23.003.
In practice
Some countries register or block devices by IMEI, and some IoT platforms lock a SIM to one IMEI so that a stolen SIM cannot be used in another device.