Из A320_Operating_Manual:
COCKPIT DOOR toggle switch:
UNLOCK position: This position is used to enable the cabin crewmember to open the door. The switch must be pulled and maintained in the unlock position until
the door is pushed open.
NORM position: All latches are locked, and EMERGENCY access is possible for the cabin crew.
LOCK position: Once the button has been moved to this position, the door is locked; emergency access, the buzzer, and the keypad are inhibited for a
preselected time (5 to 20 min).
То есть, достаточно было FO нажать LOCK, и в течение 5-20 минут даже звонка не поступит, т.к. и звонок, и полностью цифровая клавиатура заблокированы. Эта загадка, все-таки, разгадана.