To use the functions 

to prevent unauthorized persons from using your telephone

to use another telephone like your own

to change your call number

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

Lift the handset.  

Enter the code.  

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". An authorized party can also lock and unlock your telephone.