Your phone's Bootloader cannot be officially unlocked. Go Q&A, Help & Troubleshooting Guide [for beginners] - How To Unlock Bootloader via Flashtool by sid. Can An Unlocked Iphone Become Locked Again. 8491.
Quote: Originally Posted by sid.8491 I am assuming your phone can be bootloader unlocked and you have unlocking key from sony. If you don't know then refer to this post WARNING: IT WILL WIPE YOUR PHONE DATA SO MAKE BACKUP 1. Install Flashboot and Fastboot drivers of Flashtool (setup file will be on C: Flashtool drivers or similar place).
Turn on your phone, and connect with usb debugging on. Go to Plugins->Bootloader Unlock option, start it. Microsoft Lumia 640 Unlocked Phone. Now it will ask to disconnect your phone and reboot in FLASHMODE. Again reboot your phone in FASTBOOT mode. It will show a window with your IMEI number, insert your unlock key and click 'unlock'.