While you create a Route Pattern Hunt Pilot if a Directory number (DN) is assigned to the pilot, the Number already assigned error message shows up on the window to which the number is already assigned. None of the devices are found to use the DN which is assigned to the Hunt Pilot. The same DN can be used in order to set up an extension and it works fine.
In order to resolve this issue, check the Route Plan Report. If the Route Plan Report shows an unassociated DN with the same number, then delete it and the addition of the new pilot ideally works correctly.