From CUCM 12.an see the last restoration status on the phone phone page. Those information is controlled by a Cisco RIS Data Collector service parameter called RIS Unused Cisco Callmanager Device Store Period
Maybe I didn't ask the question correctly. I am interested in finding devices that have been unregistered for more than a month. I think that by default, after three days in which a device is unregistered, it is declared as 'none' by the Call manager. Since the cluster is on version 11.5, is there such a possibility?