This object specifies the backward signal instructing the network to play a Special Information Tone (SIT), the distinctive three-tone sequence used ahead of a recorded announcement, toward the calling party. This signal is what triggers the caller-facing audio cue that typically precedes messages like 'the number you have dialed is not in service', so its presence in a call flow is a direct, audible signal to the caller that something about the call could not be completed as dialed. When a dialed number is out of service, the terminating exchange sends this backward signal so the calling party hears the SIT tone immediately before an announcement plays.