You can put your call number on every other available telephone when it is set up (ask System Support).
Your previous telephone then receives the old call number of your new telephone. The call number together with the settings (e.g. programmed keys) of the telephone are changed. 

Precondition: Your old and new telephone are the first telephones at each connection. The telephone are in idle state. 

The following procedure is carried out on the new telephone. 

Press the key. The LED lights up. 

Select and confirm. 

Enter the code. 

Enter your own call number. 

Enter code (telephone lock) .
(This is not necessary if you have not determined a code yet). 

Confirm. 

Enter the code. 

If you exchange call numbers from different system telephones, all programmed keys are replaced with the default assignment. You can, however, connect your telephone to another connection and carry out the procedure.