That’s it, I knew I read it somewhere. The author, Peter ???, was fairly high up in the British SIS.
The problem, though, is that a phone is a two-wire device. When you hang up, the two wires are disconnected.
The reverse is how the phone company knows to send you dial-tone - when you pick up the phone, you close the circuit, shorting your line toward the central phone switches, tripping a relay, and connecting you to the dial equipment.
When the phone is hung up, you have an open circuit and no current flows. No current - no signal.
I suppose you could modify the phone to have a high resistance circuit when hung up, allowing a closed circuit when hung up but with too little current flow for the phone switch to trigger - but now somebody is in you house modifying your phone - not part of the original premise.
Coincidentally enough, I just finished the final work on a telephone system interface for our company’s equipment…
In a standard home phone, there are two systems connected to the wires coming out of the phone jack. First there’s your ring detect circuit, which is basically just a voltage level detector. In the old days the ring detect was just a electromagnetic coil and clapper; these days it’s usually solid-state. Either way that circuit can really only be used to make the phone ring, as it’s not connected to the talk circuit.
Secondly you’ve got the voice circuit. As already mentioned in a standard phone this is physically disconnected from the line when the handset is on-hook. No matter what signal you send over the phone line, you’re not making those switch or relay contacts close without someple picking up the phone.
Now, office phones are a different matter entierly - they’ve got all sorts of special functionality built in, and some of them can be used as silent intercoms to listen without the handset being picked up. And I suppose it’s possible that some of the newer fancy electronic phones have some kind of hook logic built in to recognize a code sent over the phone line and go into listen mode silently. I’d still want to see a cite before believing it though.
If I remember correctly from Spy Catcher (written by a former SIS Officer), it was a device to spy on foreign, probably Russian (I’m speculating here, it’s been quite a few years since I’ve read it) phone systems.
So how American or British phones are wired may not even be relevant, because it was designed for other systems. And it would be fiendishly clever that the device, if captured could NOT be turned against the American phpne system.
Wait. That’s TOO clever for the CIA or the SIS…
Any experts on Communist Bloc phone systems of the 50s and 60s out there?
Ok, here are some neat counter-measures and phone tap-detectors. i do not vouch for the info or the products mentioned on the site. if you wanna be sure you can order one of the products and see whether they work or not…
http://www.spyemporium.com/product2.html
and here’s what i was talking about earlier :
http://www.spyproducts.com/PhoneMonitor1.html
scroll down to “Informer” phone and “X” phone.
The “X” phone :
"THE “X” PHONE works and looks like a standard telephone. But is also has the capability to function as an advanced remote audio monitoring device. It’s so sophisticated that you don’t even need to call in to see if there’s activity in the area; <snip> you are immediately and clearly listening to all of the audio activity in the area of THE “X” PHONE. The intruders there have no idea whatsoever that the innocuous-looking telephone over there <snip> Additionally, this sophisticated unit has the capabilities to provide unprecedented and devastating new options; however, you are warned that it is to be used only in a legal and lawful manner in compliance with all applicable laws and EC.C. regulations. Simple to use, it operates as a fully functioning regular telephone until you activate it. "
349.95$ !
luckily their order page says:
“Many of our items are sold only to qualified Federal, State and Local Law Enforcement Agencies. Qualified agencies are invited to write to us on department letterhead for special requests and bids.”
so yes, the phone exists (subject to authenticity of website) and you can buy it at 349.95
From Bigger Secrets, (William Poundstone, 1986, ISBN 0-395-53008-3) here is the principle behind the infinity box/microphone (pages 85-86):
Interesting. Basic electronics gives you a lot of power (even with only 48 V. :D).