137
MINI-MODEM (56 KBPS DATA/FAX MODEM)
For example, the digit 4 would be dialed by
combining two tone frequencies. It would use the
frequency 770 Hz from the second row, and the
frequency 1209 Hz from the first column. In another
example, the digit 9 would be dialed with the tone
frequencies of 852 Hz and 1477 Hz.
The extended DTMF characters (A, B, C, D) are
the high-end frequencies (1633 Hz) defined on some
telephone sets with a fourth vertical column of buttons.
This fourth column provides for extended PBX control
functions; the actual functions provided depend on the
PBX manufacturer’s implementation and feature set.