This object specifies the backward signal indicating the called party's line is out of service or faulty. This is a distinct failure mode from both busy, ccasBwdRSSubscriberLineBusy, and invalid number, ccasBwdRSInvalidDialedNumber; it means the number is valid and assigned but the physical line itself has a fault, so a gateway logging this signal specifically should flag it for the carrier's line-maintenance team rather than treat it as routine call failure. If a subscriber's phone line has a cable fault, calls to that valid, assigned number would return this signal rather than busy or invalid-number, distinguishing a broken line from a wrong number or a line in use.