Oops, left this off:

http://www.cisco.com/warp/public/788/signalling/net_signal_control.html


Mario A. Amaral wrote:
>Hello All,
>
>Anyone now how I can discover a tone busy of a telephone line, with a pic or
>other chip.
>The DTMF I use a MT8870, but I need to differentiate between OFF HOOK  and
>LINE BUSY.


US busy signal is presumably - dual 480 and 620 hz sinewaves,
0.5 sec on, 0.5 sec off. These are below those of the normal DTMF
row and column frequencies, and you should probably use a method
that detects both freqs.

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