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infinity transmitter

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this is for my college project. i want to demonstrate electronic phone surveillance devices.i have studied HOOK SWITCH bypass ,third wire ,phone transmitter etc. i am going to build the infinity transmitter presented at
but the transmitter shown at "talking electronics" website cannot be activated with out the (distant)telephone being rung.
so, is there any circuit i could use to stop it from ringing and pick the line after a tone is sent along with the number dialed? (just like harmonica bug) like tone decoder circuit .how can i activate the hook switch? any ideas? not that i want to do something illegal.anyway, it is impossible to place it in any telephone without the owner knowing it.


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