AASTRA DT4x2 for MD Evolution User Guide EN - Manual (Page 108)

More Features on your Extension
MD Evolution – Cordless DT412 / DT422 / DT432 108
Phone Lock
You can protect your phone from being used by others by locking it
automatically when placed in a charger or switched off. The phone
can only be unlocked with a four digit code.
Notes:
When the phone is locked, it is still possible to send all types
of alarms.
This function requires a SIM card parameter setting. The
function can only be enabled/disabled in the SIM card, not
from the phone. The code is set in the SIM card and cannot
be changed from the phone.
Unlock the phone
The Padlock icon indicates when your phone is locked.
With the phone switched on: å
Dial your four digit code.
Now you can use your phone.