Removing users but keeping number within the department

How would I go about terminating a user but being able to keep that number within the department so that we do not assign that number to a different department. For example, we have a terminated employee in Finance and when I remove that user from RingCentral, that number goes to the unassigned queue and is not labeled to which dept the phone number was previously assigned to. How would I be able to keep that number within that dept so that when we hire a new finance person, we would know and can assign that number to that dept only. Any help would be greatly appreciated.