>What about a line voltage change.  Judging by British systems this is the
>standard:
>Ring is on audio in line and is capacitive coupled to bell line
>When the phone is picked up, BT must know when and thus sense a line voltage
>change.
>
>Tim

On the side with the RINGING telephone, the polarity switches when the hook is lifted (besides, the ringing in a phone with a bell is caused by switching polarity fast). A hook-lift is signalled by changing the impedance.

On the CALLER side, there is no simple way of telling whether the recipient picked up the phone or not. My ZyXEL voice modems resorts to a "ringback goes away" timer for this (which by the way doesn't work, and nobody at ZyXEL ever answer my faxes).

Regards,
Glenn
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