How to detect ON/OFF Hook of connected Line of Exchange side

[nuttaphong191]

How to detect ON/OFF Hook of connected Line of Exchange side.
I want to detet ON/OFF hook status of line connected to exchange side
If I dial a number and picking the Handset of the called party, I should able to detech whether the Handset is picked or Not

Can any body help and if possible give the circuit

Thanks
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[torbjorn]

The best way is to check if any line current is flowing. In a typical two-wire telephone system, a current between 7 mA and 120 mA can be expected when the subscriber set is off-hook, and almost zero (only leakage currents in the order of some uA) when it is on-hook.

There are so-called line current relays that have traditionally been used for such purposes (especially in Germany where all modems, faxes etc formerly were required to monitor line current and not try to go off-hook if some other set was already using the line). Most companies manufacturing reed relays have some types that are intended as line current relays.
Remember to connect a bipolar 100 uF capacitor parallell to the coil to bypass the speech (AC) currents.

More modern circuitry often uses optoisolators, but those have the drawback of a higher and non-linear voltage drop. This may cause distortion and possibly other problems, especially on very long lines or in countries where the telecom regulations require especially low voltage drop across the subscriber set (i.e. Brazil and Malaysia).