How can people listen in to conversations in a room via a plugged in telephone?

September 18, 2010 9:42 PM

How can people listen in to conversations in a room via a plugged in telephone?

I was reading Body of Lies by David Ignatius, and twice within the book, Ferris unplugs the telephone from the wall and removes the handset from the base in order to keep his conversation between himself and another person in the room confidential. It is not stated whether the phone was corded or cordless. But both individuals are having a conversation in the same room and the phone is unplugged and taken from the base as a precaution. This doesn't seem to be a case of a "phone tap" or bug placed inside the phone, though he never specifically says there is no bug within the telephone. (I think a "bug" would not need a phone unplugged and taken from its base to be deactivated anyway.)

I am wondering by just what mechanism this would work. I can understand that the entire phone itself could somehow work as a "microphone" and someone could listen in while it is plugged in provided they have some device to intercept the conversation. But removing the handset from the base would then seem to make it easier to listen in (because that's one the of steps you take to "activate" a phone--you take it from its base--barring speaker phone) or else superfluous once the phone is unplugged.

If anyone could shed light on this, I'd be very grateful. David Ignatius writes very credibly with realistic tradecraft, but I have never heard of this means of spying before.

posted by LillyBird to technology (17 answers total)

Let's assume he's writing a realistic character. If the phone is unplugged from the wall, there's no way the phone can be used as a listening device (barring some kind of wireless battery-powered device, but then that could be hidden anywhere.) So what I'll propose is this: the character is written as unplugging the phone to prevent eavesdropping, and removes the phone from the cradle to remind himself that he's unplugged the phone. A very reasonable and human thing to do, if you're going to be unplugging the phone and don't want to forget to plug it back in.
posted by davejay at 9:54 PM on September 18, 2010 [1 favorite]

Well, there are several ways this can be done. One is to plant a wireless bug in the handset which draws power from the phone line. One could do something similar with a battery powered bug, but it wouldn't be able to last very long. However, with the battery powered bug, unplugging the phone or removing the handset from the base would have no effect.

Another option is to disable the hookswitch on the phone or make it so that it can be controlled remotely. When the hookswitch is not depressed, the phone's microphone is also activated and could be listened to by anyone physically tapping the phone line. Again, unplugging the phone would disable the "bug."

So it both makes some amount of sense and is a little oddly described.
posted by wierdo at 9:55 PM on September 18, 2010

Could you provide a quote?
posted by clorox at 9:56 PM on September 18, 2010

In which time period is this book set?

Some of the earliest bugs involved utilising the microphone and line of an existing telephone. A fair bit of Spycatcher, for example, involves breaking into places to fiddle with people's (wired, naturally) telephones.

I'm only guessing, but I suppose the uncradling the handpiece (like weirdo says) is because that switch is also used to switch the bug off while the phone is being used, or adjust the sensitivity of the mic or something similar. If the listener had the very faint signal strongly amplified - to be able to hear conversations in the room, and then someone used the phone, there may be issues (which, yes, would seem to be superfluous presuming that the telephone line is being used to transmit the signal).
posted by pompomtom at 10:11 PM on September 18, 2010

In old (ringer) telephones, you could eavesdrop on conversations in the room without an extra bug.

The ringer is basically a magnet, a coil, and a hammer which strikes the bells. The coil was hooked directly to the phone line, so an alternating current would cause it to magnetize/demagnetize and strike the bell accordingly.

Well, the coil and hammer can also be used as a microphone. When someone in the room talks, the hammer vibrates, inducing a current in the coil. Probably not high fidelity, but good enough for spying?
posted by Laen at 10:58 PM on September 18, 2010

Some phones are usable as intercoms. Its been a plot in a few murder mysteries (on tv) where someone has altered an intercom phone so that the intercom is either turned on all the time or can be turned on but still look like its off (removing the led indicator)
posted by missmagenta at 11:01 PM on September 18, 2010

You can do amazing things with extremely faint or noisy signals if you apply the right technology. In this case you have a live circuit attached to a microphone that may not be live but still will vibrate if sound hits it. Those vibrations could cause miniscule fluctuations in the live circuits of the phone & in turn the signal constantly present on the wires of the phone. If you had a sensitive enough signal analyzer at the other end it's conceivable that you could extract the distortions caused by the dormant mic & ultimately reconstruct the sounds that caused them.

For some related capabilities, look up NSA's programs called NONSTOP & HIJACK.
posted by scalefree at 11:18 PM on September 18, 2010 [1 favorite]

In old (ringer) telephones, you could eavesdrop on conversations in the room without an extra bug.

In fact there was a great AskMe comment that I was just searching for (and failed to find) about someone growing up in the USSR (or possibly the DDR) and becoming aware due to oddities of the ringers of the phones in the house that the state was using this method to monitor the household for subversive conversations, and how they ended up defeating it/working around it.
posted by Rhomboid at 12:18 AM on September 19, 2010 [1 favorite]

Tangentially related is the area of side channel attacks.
posted by ollyollyoxenfree at 2:20 AM on September 19, 2010

I used to have a Panasonic answering machine that had a listening system built into it. You could disable the ringer on the phone and with your personal code, select "room listening mode". You could hear anything that was going on in the room.
posted by JohnE at 5:48 AM on September 19, 2010

In which time period is this book set?

Definitely this. For one thing, "unplugging the phone from the wall" might be an anachronism. Before about 1960, phones were hard-wired into the wall, and disconnecting them required a screwdriver.
posted by Chocolate Pickle at 7:48 AM on September 19, 2010

I used to have a Panasonic answering machine that had a listening system built into it.

Science fiction author Bruce Sterling used to have one of those too. It worked well enough that you could hear the score of a football game on the TV on a Sunday afternoon. Or so I've been told.
posted by scalefree at 10:31 AM on September 19, 2010

Wow! Thanks everyone...so many good explanations. You guys are amazing. I appreciate all the responses, even if they did not seem to be the answer to this precise situation.

wierdo & pompomtom: I think you might have nailed it. This explanation seems very likely, given his actions. And thanks so much for supplying the word "hookswitch." I had no idea what to call that "button." But now it seems odd that they'd have a bug that could be "turned off" when the phone is picked up--wouldn't you want to tap the phone calls as well? Or maybe it's as pompomtom suggests adjusts sensitivity or something...

pompomtom & Chocolate Pickle: The book is set in modern day, after 2001. At least one of the scenes takes place in Amman, Jordon. It's in the apartment of an American who works with the NGO doing Palestinian relief work. She has a cell phone, but I can't be sure what type of "regular" phone she has in her apartment, though it would seem to be modern, as well.

clorox: Here's one of the quotes: "He closed the door and retreated deep into the bedroom. He reached for the phone at Alice's bedside and detached the handset from the base, and then unplugging the phone itself from the wall. Nobody should overhear what he was about to say. He sat down on the bed and motioned for Hani to sit next to him."

scalefree & ollyollyoxenfree: I'll definitely look into the NSA's programs and side channel attacks. Thanks!

Thanks again, everyone! I'll also read Spycraft, which pompomtom mentioned.
posted by LillyBird at 10:43 AM on September 19, 2010

A normal phone mechanically disconnects the speaking and listening portion of the phone when it is on-hook. All that is connected is the bell. Unless you are yelling at the phone, the hammer won't pick up any vibrations. It is too heavy.

So it would have to be a tampered-with phone, as well as a tampered-with phone system along the line somewhere.

And if someone was going to do that, why not bug the room in a less easy to defeat way?
posted by gjc at 3:28 PM on September 19, 2010

So it would have to be a tampered-with phone, as well as a tampered-with phone system along the line somewhere.

The air gap between the mechanical hook switch contacts creates a makeshift capacitor. If you inject a high frequency signal (on the order of several MHz) on the line you can overcome that capacitance and create a circuit with the microphone of the handset in series, even though it's on-hook. You can then look at the modulations of that high frequency signal which will correspond to audio in the room. At least that's one of the theories put forth by comments on this post.
posted by Rhomboid at 5:16 PM on September 19, 2010

It doesn't have to be anywhere near that complicated, you don't need bugs at all. Here's what I would do. Say the phone is in a conference room where people will be talking, I would dial my mobile phone from the landline phone, put it on speaker and then replace the handset. Walk out of the room, and when other people are in there doing their thing, you can hear it all...
posted by Jubey at 10:49 PM on September 19, 2010

I should add, you can hear it all from the mobile phone, in case it wasn't obvious.
posted by Jubey at 10:50 PM on September 19, 2010