Brother MFC990CW Service Manual page 376

On the machine (base unit)
(6) Press the Stop/Exit key.
The machine returns to the ready-to-register state, showing the "REGISTER HANDSET"
on the LCD.
(7) Press the Stop/Exit key again.
The machine returns to the initial stage of the maintenance mode.
-- Deregistering all cordless handsets --
(1) Press the 3 key twice in the initial stage of the maintenance mode. For models without
numerical keypad, enter 3 twice in this order using the
The "REGISTER HANDSET" appears on the LCD.
(2) Press the Erase key.
The "COMPLETED" appears on the LCD and deregistration of all cordless handsets is
completed.
(3) Press the Stop/Exit key.
The machine returns to the ready-to-register state, showing the "REGISTER HANDSET"
on the LCD.
(4) Press the Stop/Exit key again.
The machine returns to the initial stage of the maintenance mode.
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and OK keys.
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