The International Mobile Equipment Identity (IMEI) is a pre-recorded USSD code on GSM mobile phones. This code uniquely identifies the device worldwide and is transmitted by the device to the network upon connection.
This means, among other things, that the operator we use not only knows who is making the call and from where (SIM), but also which phone device it was made from. The operator can use the IMEI to verify the device's status using a database called the EIR (Equipment Identity Register).
Overview
The IMEI allows for functions such as blocking mobile devices in the event of theft. To do this, the user simply needs to notify the phone's IMEI to the telephone operator so they can block it and thus prevent calls from that device by blocking calls.
It's also possible to unlock a locked mobile phone for use with a new mobile operator using the IMEI code, regardless of the make or model. A device's IMEI is usually printed on the back of the device, under the battery. In the case of devices with an internal battery, it can be found on the back of the device in small numbers, on the bottom of the device.
In some cases, it can also be found on the SIM card slot. You can also dial the USSD code *#06# (asterisk, pound, zero, six, pound) to display it on the device's screen. Devices with two SIM slots have two different IMEI numbers, one for each SIM slot.
It is unclear whether local barring of IMEI has any positive effect, as it may result in international smuggling of stolen phones.
EIR
The EIR (Equipment Identity Register) is a database containing information on the status of mobile phones. Within this database, there are three IMEI lists: the white list, the gray list, and the black list.
The whitelist: identifies the devices authorized to receive and make calls. This list must always exist in the EIR, even if it's the only one, the other two are optional.
The Greylisting: identifies devices that can make and receive calls, but can be monitored to discover the user's identity using information stored on the SIM card.
The blacklist: identifies devices that are blocked from connecting to the network. It contains the identifiers of stolen or illegally used devices, as well as those that cannot access the system because they could cause serious technical problems; therefore, they cannot make or receive calls.
