I am trying to unlock my Sony Xperia XZ Premium's bootloader as I am wanting to use EE (UK)'s VoLTE service (it only works on EE firmware and the device I bought from CEX (UK) second hand has Vodafone UK's firmware on it so therefore won't work). However, in the phones' Service Menu, it says the bootloader cannot be unlocked despite OEM unlocking being enabled in the Developer Options menu. Is this a mobile operator problem or is my device faulty. Please help. Thank you!