Here are some frequencies used in the=20
phone system :
=20
Tone         frequency (Hz)  Seconds
                              On/Off
---------------------------------------
Dial         350+440           all/0
Busy         480+620           .5/.5
Ringback=20
  Norm:      440+480             2/4
   PBX:      440+480             1/3
Congestion   480+620            .2/.3
Reorder      480+620            .3/.2
Receiver off 1400+2060 =20
=20
Here are the frequencies of the touch=20
tone sounds. NOTE : they are a
combination of TWO TONES. I have also
included the tones of the A B C and D=20
keys not normally found on a phone.
=20
Key       Tone #1        Tone #2 (Hz)
--------------------------------------
1          697             1209
2          697             1336
3          697             1477
A          697             1633
=20
4          770             1209
5          770             1336=20
6          770             1477
B          770             1633
=20
7          852             1209
8          852             1336
9          852             1477
C          852             1633
=20
*          941             1209
0          941             1336
#          941             1477
D          941             1633
=20
Each combined tone must have a minimum
duration of 50 milliseconds and have
a 50 millisecond break between it and
the next tones to be recognized by=20
the phone company.
=20
=1A