To unlock the phone, you need to use the following steps:
- turn off the phone
- press Volume Up and Volume Down and Power buttons at the same time
- wait until the service menu appears
- select Wipe Data/Factory reset
- confirm with Power button
- wait for the phone to boot
Note that it will take a while to boot. If the phone gets stuck at the boot screen, turn it off then remove the battery for a few seconds and put it back.