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If I'm right you need a device connected to the telephone line that tells you when the telephone hang on.
It sholud be easy because normally when the line is free its voltage is around 40 and when the telephone is hang off the voltage is much lower (less than 12V).
Beware that when the ring signal is applied to the line from the Central office to signal an incoming call the line voltage can reach 200V
I suggest you to use a simple op-amp, to get and recognize the voltage level of tel line.
If it is less than 15-20volts, you can detect the hanging off.
Good Luck.
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