To solve invalid IMEI issues . Generally this problem occurs when you factory restores your phone or wipe factory data through recoverys. There are two methods -
1) Engineering mode" process- Sorry cant explain dis process its against the rule".
2) By using MUT(Mobile Uncle Tools) Works on Rooted Android Phones.
Before doing Factory restore first download MUT software from any external website and kip the apk file on your SDCard dont remove it or you can kip it on your PC and move it later on to your Phone via usb. Then go to SETTINGS of phone then SECURITY then tick the UNKNOWN SOURCES option and then install the downloaded MUT.apk open the software there is an option called "IMEI Backup". Backup this in the memory card or SDCARD of your phone(*Note- Backup the IMEI in the memory card of the phone or else it might get removed while doing factory restore). You can check the backup file through any filemanager in the SDCard to be sure if its there or not, its just a small file.
Then do the factory restore. After starting up your phone again install the MUT software and go to the IMEI Restore option and hit it then reboot it. Your phone will gain its own IMEI and the problem will be solved.
Thank you !!! Hope it works.....