Create a Call Handler with the desired greeting and an after greeting action of hang up. Then create a CTI RP & DN that is CFA to VM. Either that DN or a Translation Pattern needs to wildcard match unassigned numbers. Most customers that implement this choose to only assign this pattern to incoming trunk CSS, ie only incoming external calls will hear it.
Be aware that spam/robocalls can chew up a lot of CUC’s ports if you have large DID blocks with a bunch of unassigned numbers.
