SIT tones are the "Special Information Tones" that are used in the telecommunications industry to facilitate the communication of a message to a computer or similar device. In the United States and some foreign countries these are heard as three quick tones sounding like "doo....duh....dah" and followed by a message like "The number you dialed is no longer in service". There follow chart lists all the distinct SIT tones that are used:
Period Frequency Designation
SSL LLL IC - Intercept - Vacant No. or AIS or etc.
LLL LLL NC - No Circuit (Inter-LATA carrier)
LSL HLL VC - Vacant Code
SLL HLL RO - Reorder Announcement (Inter-LATA Carrier)
LSS LHL #1 - Add'l Reserved Code
SLL LHL RO - Reorder Announcement
SSL HHL #2 - Add'l Reserved Code
LLL LLL NC - No Circuit, Emergency or Trunk Blockage
Key: Period: S = Short 274 msec L = Long 380 msec
Frequency: L=Low 913.8 hz 1370.6 hz 1776.7 hz
H=High 985.2 hz 1428.5 hz
The tones above are used by CDA (Call Disposition Analyzers) to distinguish between a human voice and a recorded announcement and to categorize the type of announcement. Most CDAs just look for three sequential tones occurring in less than 3 seconds by using a bandpass filter centered around 600 Hz.
In practice the most common use for SIT tones is to help predictive dialers (computer automated dialers used in telemarketing). When the dialer "hears" the SIT tones it can take action. If it hears the VC series they are typically set up to remove the callers number from the dialing list as it usually means the phone has been disconnected for good.
Another use for the SIT tones is to combat annoying telemarketing calls. Products like the TeleZapper play the VC tones whenever you pick up a line. You can also download these tones from the internet and put at the beginning of your outgoing message on your answering machine and save yourself the $40. ("2002.11.16@15:36 yerricde says ...this won't work on some digital answering machines, as they are designed only for voice and don't pass pure tones properly. My Uniden answering machine has this feature".)
Go to http://artofhacking.com/files/sounds/index.htm to listen to or download all the SIT tones.
It might seem redundant to call these SIT tones since the "T" in SIT stands for the word "tones", but rest assured for some reason this is usually what is said when someone wants to discuss special information tones.
The same phenomenon is found with other acronyms too. In fact my own bank is called UMB Bank and the UMB stands for "United Missouri Bank" (or it least it used too). When the bank went multi-state they switched to the acronym UMB, but still added the word bank.