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The Line Busy Tone indicates that the called customer's line has been reached but that it is busy or being rung or on permanent signal. When a line busy signal is applied by an operator, it is sometimes calls a busy-back tone.
Line Busy Tone is Low Tone on and off every .5 seconds.
(Low Tone is 480 Hz and 620 Hz at -24 dBm0/frequency.)
To detect Line Busy Tone with MCU, you have to filtered the tone and than detect on and off signal every .5 seconds