23. What are the DTMF frequencies?

DTMF stands for Dual Tone Multi Frequency. These are the tones you get when
you press a key on your telephone touchpad. The tone of the button is the
sum of the column and row tones. The ABCD keys do not exist on standard
telephones.

         1209 1336 1477 1633

        697   1    2    3    A

        770   4    5    6    B

        852   7    8    9    C

        941   *    0    #    D

24. What are the frequencies of the telephone tones?

Type                Hz          On      Off
---------------------------------------------------------------------
Dial Tone         350 & 400     ---     ---
Busy Signal       480 & 620     0.5     0.5
Toll Congestion   480 & 620     0.2     0.3
Ringback (Normal) 440 & 480     2.0     4.0
Ringback (PBX)    440 & 480     1.5     4.5
Reorder (Local)   480 & 620     3.0     2.0
Invalid Number    200 & 400
Hang Up Warning 1400 & 2060     0.1     0.1
Hang Up         2450 & 2600     ---     ---