There are two modes available for maintenance operations: Recovery mode & Download mode. The download mode is used to flash the firmware of the phone using custom or third party tools. You need to access the Recovery mode to be able to unlock it. Press Volume Up + Power to turn off the phone and keep them pressed until you enter a menu with multiple choices. Choose Wipe Data/Factory Reset and reboot the phone. After it boots, the phone should be unlocked and ready to use. Initial configuration needs to be followed before you can use it.