You can apply to your carrier for "call tracing" on malicious calls. An authorised extension is then in a position to request the identification of the phone number.  

This also works for 30 seconds after the malicious caller has hung up. However, you should not hang up. 

This function can also be called by entering the code (). 

You receive a malicious call.
Do not hang up. 

Call the system menu. 

Select and confirm the menu item. 

Select and confirm the menu item. The caller is identified in the attendant console. 

Press the On-hook key to end the process.