User guide

22 RealCT Direct API Developer Guide

Chapter 2: Digit Handling

Assigning Frequencies

The forward and backward signals use a different set of six

frequencies for a total of 15 possible forward and backward

tones.

Forward frequencies: 1380, 1500, 1620, 1740, 1860 and

1980 Hz

Backward frequencies: 1140, 1020, 900, 780, 660 and 540

Networks can also operate using only five forward frequencies

for a total of ten signals, and four or five backward frequencies

for a total of six or ten signals. Mexico and China, for example,

use only four backward frequencies.