Where do you go to find the "next available" DN to assign? Is that managed via a spreadsheet and is that up to date with what's in CUCM?
From our 15+ years now of working exclusively with Cisco collaboration (beginning in the "CallManager" days) and on lots of feedback from our users and partners, one of the limitations with CUCM is that it's difficult for an administrator to easily identify that a directory number (such as "10111") is available for assignment, meaning it doesn't yet exist in CUCM. Or it does exist, but it can be reassigned.
Since Directory Number 10111 doesn't appear in the following sample screenshot from CUCM Admninistration, it's available to be assigned - but how quickly can you see that when you need to?
Because it doesn't exist in CUCM, this directory number (and any others which don't exist) will simply not appear in any CUCM report or search, leaving you to have to somehow identify the "gaps" to reveal which numbers are available. Sure, it's doable, but you likely have many other things begging for your attention.
Quickly identify which directory numbers are available, inactive/unassigned, and/or active as well as shared.