IMEI stands for International Mobile equipment identify
It is a number, usually unique, to identify 3GPP and iDEN mobile phones, as well as some satellite phones. It is usually found printed inside the battery compartment of the phone, but can also be displayed on-screen on most phones by entering *#06# on the dialpad, or alongside other system information in the settings menu on smartphone operating systems.
GSM networks use the IMEI number to identify valid devices, and can stop a stolen phone from accessing the network. For example, if a mobile phone is stolen, the owner can have their network provider use the IMEI number to blacklist the phone. This renders the phone useless on that network and sometimes other networks, even if the thief changes the phone’s subscriber identity module (SIM).
Some times people run into trouble of INVALID IMEI after resetting their phone or tablet to the factory settings or after flashing.this is because the imei Was wiped in the process
If the IMEI is invalid you wouldn’t be able to make a call or send messages or connect to the internet, the device will only get Network signals
Here’s how to fix imei on Spreadtrum Android devices such as Itel, Lenovo etc
Download SPD IMEI Repair Tool on a Windows PC and execute the app. The instruction notes will prompt. Enter the IMEI number and click the save option. Reboot the device and once started, the tool will replace the IMEI number.