AASTRA DT4x2 for BusinessPhone User Guide EN - Manual (Page 71)

71 BusinessPhone – Cordless DT412/DT422/DT432
Security
Lock/Unlock the Cordless
You can protect your Cordless from being used by others by
locking it automatically when placed in a charger or switched off.
The Cordless can only be unlocked with a four digit code.
Notes:
When the Cordless 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 Cordless. The code is set in the SIM card and
cannot be changed from the Cordless.
Unlock the Cordless
The Padlock icon indicates when your Cordless is locked.
With the Cordless switched on: å
Dial your four digit code.
Now you can use your Cordless.