From a couple of folx....
Actually, there is a germ of truth in this. On older phones (don't know if this works on newer electronic phones) when the handset is 'on-hook' a switch opens and breaks the voice circuit. This of course only works for DC circuits. If you drive that same circuit with an AC signal . . .
There's another angle I may have mentioned before. Many electronic phones come with a ``feature'' that allows you to call home, produce an electronic tone and eavesdrop on your own house. When the tone is sounded, the ringing stops (or never starts) and the phone goes into ``off hook'' mode (i.e., the microphone in the mouthpiece is turned on).
[ Snip! ] A simpler solution is to keep the phone in another room used mainly for phonecalls, or even in a small office if you don't make an sounds there worth evesdropping on. (The really paranoid can soundproof/tempest-proof the room....)