Reasons for NOT moving the DID number with the individual:
Bob moves from Accounting to Marketing. If I allow him to take his DID with him, future conversations go like this: "Hi Bob. Can you check on purchase order 123456 to see if we're up-to-date on the invoicing?" "Well, Joe, I'm no longer in the Accounting department, so I don't have access to the AP system. Let me transfer you to the new person. Their phone number is 123-456-7890." "What was that number again? I'm going to have to update all of our contact sheets."
I think that each organization will be different in how they approach this. For me, DID numbers are associated with a position or a department, not an individual.
Susan
"When the gods wish to punish us, they answer our prayers." - Oscar Wilde, An Ideal husband, 1893