To use the functions 

to prevent unauthorized persons from using your telephone

to use another telephone like your own

you need to enter a personal identification number, which you can save yourself. 

Press the key. The LED lights up. 

Confirm. 

Enter the PIN. 

Enter the current five-digit PIN.
If you have not yet assigned a PIN, use the PIN "00000" the first time you enter it. 

Enter the new PIN.
 

Repeat the new PIN. 

If you forget your PIN, contact System Support, who can reset your PIN to "00000".