[Q] how to unlock bootloader of a branded xperia arc

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hy guys

i have orange branded xperia arc
i want to remove its branding, for that purpose i flash the 3.0.A.2.181 generic firmware in it using flash tools... the orange logo disappear while its starting but
still i have a problem
and that is...
i want to connect my phone with computer to unlock its bootloader but while i holddown the menu button and connect the usb cable its turn on and start charging.
no blue light notification comes
so please tell me how can i totally remove its banding or how i can connect it in that mode

or simply i can ask how can we connect a branded phone in fast bootloader mode

At this point in time, we are not able to enter into fastboot mode with a sim-locked phone. Changing the firmware or unlocking the sim doesn't allow us to enter fastboot. Right now all we can do is wait

And what about a branded Arc WITHOUT sim lock?
Is that one bootloader unlockable (after debranding)?