first format using SP flash tool then flash again after flash has done do not start the phone i repeat DO NOT TURN ON THE PHONE!!!!now unplug the usb cable then run any imei writer like IMEI&SN Writer V1.5.3 or maui meta then just write your imei that's it now boot the phone there you go and this will work %100 i'm sure if it doesn't work then it means you are doing something wrong
also i have been trying to figure how to fix this imei invalid problem for 1 week but nothing worked i have tried the several tools and ways like IMEI&SN Writer V1.5.3, Maui meta, engineering mode and SN_Station nothing worked and then i tried this AND IT WORKED LIKE A CHARM
my device: Symphony W85 (MTK6572)
if it helped don't forget to hit the "thanks" button