Telco approval is a problem for this kind of thing.
One possible solution - IIRC the 'off-hook' state is signalled by drawing a
current from the line. If you can detect that current without actually being
connected to the line itself, you can do this without requiring Telco approval.
Looking up the minimum current which signals the device as off-hook, and
coming up with a sane and reliable method of detecting said current, is left as
an exercise for the reader.
But we're drifting very off-topic.
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