Originally Posted by Nidhal AKA the king
BTW: Bootloader unlock allowed: No
(is it possible to make it Yes somehow?)
No, no way.
To debrand your phone use latest flashtool and flash a FTF from Mega Collection for your Model C6903, there is no any issues as they are Stock and made for your phone. The R1B or something else is just a customization, not even noticeable.
Flash Generic/International and you are Fine.
IF I HELPED DO NOT SAY THANKS, CLICK IT.
My Evolution: Sony Xperia Z1 - UB + 4.3 Rooted + Deodexed, HTC HD2, HTC Diamond 2, HTC HD Blackstone, Nokia 7070 Prism, Qtek 9100, Nokia 3100, Motorola T720i, Nokia 3310, Sony Ericson PF 768 (Blue)
How Get Root on Sony Xperia Z1 with Locked Bootloader and the Latest Android 4.4