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>You could get rid of the button by detecting on/off hook. On hook is about
>48V DC, off hook is much less than 15V. A high impeadance voltage divider
>and a comparitor can easily detect the voltage. Be sure though to clamp
>down ringing voltages which can get up to 150V.

More dependably, use an opto and bridge rectifier to measure current into
the phone.
The voltage method works, but the ranges are wider than you think, they
actually overlap.

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