LevelOne VOI-8001 - Manual (Page 22)

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 Busy: Define the busy tone for generate.
 Ring: Define the ring back tone for generate
b. Low freq: Lower frequency for defined tone
c. High freq: Higher frequency for defined tone. Each tone can define two
frequencies, if only one frequency needed, please leave High Frequency to
0.
d. T_ON_1, T_OFF_1, T_ON_2, T_OFF_2:
 The cadence pattern of up to four intervals for each dual-frequency.
Minimum Cadence value is 30msec.