- Sony requires you to request an unlock code from their [Unlock Service]. They can pull the plug anytime. You have to send your IMEI and agree to warranty voiding.
Some carrier locked and US devices can never be unlocked. On Sony devices ([but not all?][service-menu-gone]) you can check if the bootloader is unlockable using the service menu.
1. Dial `*#*#7378423#*#*` in the dialer.
2. Tap "Service Info", then "Configuration", then "Rooting Status".
3. Under Rooting Status look for "Bootloader unlock allowed"