This object specifies the backward signal indicating the dialed or called number is invalid or not in use, such as an unassigned country code, unused trunk code, or unallocated subscriber number. This is the R2-level trigger for what a caller experiences as an invalid-number announcement, so tracking how often this specific signal fires on a trunk can reveal misdialed-number patterns or stale routing tables pointing at decommissioned number ranges. A call dialed to a subscriber number that was deallocated last month would cause the terminating exchange to return this signal, prompting the originating gateway to play an invalid-number announcement to the caller.