Honestly, not... But you don't seem to understand the issue. Calls are not going to mailbox 9999 (unless your fax extension was literally 9999), they are going to the general auto attendant, then going to Mailbox 0 and the extension associated with that mailbox.
Why not simply make it so the fax extension does not forward to VM (it's a simple tenant change), or if it's not being used, just delete the extension entirely?
I guess you could set digit restrictions to 1, 2, 3, 4, 5, 6, 7, 8, 9, 0 and that would accomplish the same thing but your essentially disabling the auto attendant.