IMEI Invalid - Lenovo S860, rooted android 4.4.2, nothing works
I am using MTK Engineering Mode app from Play Store to restore my lost IMEI after flashing ROM. You should know your previous IMEI number. It is usually mentioned in the area underneath battery. When you will open your phone you will see it. 

Steps:


Open MTK Engineering Mode.
Click MTK setting.
From connectivity tab choose CDS information, radio information, phone1 or phone2, send AT command AT+EGMR=1,7,”your15digitsIMEI”.
Click send AT command button. 
Restart your phone and it should work now. 


If still unclear see my article here.