Touch-Tone Frequencies
----------------------

     1209 1336 1477 1633
697  1    2    3    A
770  4    5    6    B
852  7    8    9    C
941  *    0    #    0

Each touch tone is a combination of two tones. For instance, 3 is 697 hertz and
1477 hertz. This diagram also contains the four extra tones that every touch
tone phone is capable of producing. These tones are used in the U.S.
military phone network (AUTOVON) for establishing the importance of the call.
We'd like to hear specifics of any further uses for them.

Special Information Tones (S.I.T.)

We've all heard these. They're the special tones you get right before you hear
a recording telling you the number you've reached is out of service. They're
also used for a multitude of other conditions. The purpose of these tones is to
permit an automatic Call Disposition Analyzer (CDA) to differentiate between a
human voice and a recorded announcement, and to categorize the type of recorded
announcement.

Special Information Tones are a series of three tones at the beginning of an
intercepted call.

SIT Tone type and usages

Period    Frequency    Designation
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SSL       LLL          IC - Intercept - Vacant # or AIS, 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 - Additional Reserved Code
SLL       LHL          RO - Reorder Announcement
SSL       HHL          #2 - Additional Reserved Code
LLL       LLL          NC - No Circuit, Emergency, or Trunk Blockage

Period duration: 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)

This information was taken from a central office recorder/announcer
installation manual circa 1983.