lauren@vortex.UUCP (10/15/85)
The set of three tones you hear on many recordings these days are called "Special Information Tones." They vary for different sorts of situations (e.g. unassigned number, all circuits busy, etc.) They use a variety of rather oddball tone frequencies that were detailed in an issue of BSTJ some years back. These tones can be used by automated line/trunk testing equipment to determine the termination status of calls. --Lauren--
lauren@vortex.UUCP (10/17/85)
I hate to contradict someone at an AT&T site, but the SIT's are NOT MF (blue box) tones. They are a bunch of odd frequencies that are unrelated to both MF and DTMF (touch tone). I remember noticing that they were a set of really "strange" frequencies when I read the original BSTJ article regarding them. --Lauren--