are you guys SURE you know what you’re talking about?
i believe the OP is not talking about just unlocking the bootloader, which is easy to do using sony website and fastboot, after checking OEM unlocking.
my comments are about removing the carrier-specific bootloader-unlock-restriction that, as far as i know, is present for sony xperia models ONLY on japanese-branded, docomo-carrier phones.
this restriction results in Bootloader Unlock Allowed: no in the service info menu:
- android dialer => dial
*#*#7378423#*#*=> Service info => Configuration => Rooting Status
this absolutely prevents fastboot from unlocking the bootloader, with or without the sony code, and with or without oem-unlocking enabled. the only way to remove that is with some computationally intensive cracking that you used to have to use the S1 unlocking tool to do, and as far as i know, is now impossible.
@Chranah, if you dial that code in the dialer, go to service info/config/rooting, do you see Bootloader Unlock Allowed: no if it actually says yes, then this should be easy.
ALSO, its possible this is somehow different for the 10 III. maybe docomo stopped with the unlock-restriction. the last device i had to do this for was a docomo-branded XZ2 compact.