If it is a published number, you probably want it to go somewhere between that person leaving and the next person taking on the role. Cisco is working on a feature to assist with that, but, in the meantime, the easiest thing to do would be to create an Auto Attendant or Hunt Group (or any Feature, really) that does something with the call. It could just forward it to another number, or it could play an announcement, or whatever makes sense, but attaching that number to the feature while it is not in use should give you some idea of what it was used for.
Hunt Groups, for example, can have up to 11 phone numbers. You could create a Hunt Group called "Vacant Sales DIDs" and when anyone leaves the Sales team, their number goes there and all vacant sales numbers go to the same place. Then you can find a vacant Sales number when you need one. Repeat for every group that you publish numbers for.