IMEI. Means International Mobile Equipment Identity, it is used to identify phones on networks . If your phone’s IMEI is INVALID you wouldn’t be able to use Network services with the phone.
CAUSES OF INVALID IMEI
Possible reason for invalid IMEI error could be a problem from the MTK setup of the Android device. It could also be as a result of factory reset or flashing of the Android device without the removal of the mobile Sim network.
Whatever be the cause, we shall, without further delay, jump into the purpose of this article which is to show you how to fix invalid IMEI error on Android Smartphones .
We know that there are several methods to fix this problem but in other to make it easier, we will make use of the simple and popular  methods.
When it comes to finding solutions to this issue, knowing how to change the IMEI of your Android device is the first thing you need to know to fix this problem that has deprived you of your network offers and services.
The methods 1
3. Install and run the application.
4. Press “MTK Settings” from the first menu option.
5. Slide left to “Connectivity”
6. Next press “CDS Information” follow by “Radio Information” in the displayed menu option.
7. If your device has two SIM option, choose the preferred slot for IMEI change.
8. After “AT+” type EGMR=1,10,””
9. Place the new IMEI number you want in between the double quote eg. AT+ EGMR=1,10,”358265013803256″
10. Leave a space between the + sign and EGMR
11. Send the command and reboot your device.
12. Dial *#06# again to see the changes.
Method 2 needs root
1> With a computer DOWNLOAD and install MTK DROID TOOL ,after successful installation launch the application
DOWNLOAD and install ADB DRIVERS and DOWNLOAD and install MTK USB DRIVERS.
Having ROOTED your device connect it to the computer with the original USB cable
When the computer detects the phone and display information about it, select get root and wait to see the green indication , then select write IMEI, type the 15 digit IMEI number of your choice .
Reboot the device to apply the changes . DONE
DONT KNOW WHAT IS ROOT?