Device That Calls W/O A Ring So THEY Can Listen In?

What is the deal with the monitoring device that the feds have that allows you to call someone without their phone ever ringing, so that their private household conversations can be listened in on? How does it work?? Are they available to the public???

Is there a device that you could buy or build to detect this device on your phone line?

I doubt what you describe exists. Listening bugs are far simpler than that and what you describe makes no sense. Why would they wwant to do something like that? I do not think it is feasible.

There was such a device described in the movie The Conversation, but I don’t think it could work as described there. As I understand it, if the handset for a “traditional” phone is still on the hook, there is no electricity flowing to the earpiece and microphone. No electricity, no functionality.

Further, I agree with sailor’s comment that bugs are usually much simpler than that. Usually a small microphone with a transmitter.

It would allow “them” to listen into any house with a phone without having to physically visit and bug the place. Hit and miss in that you could only listen in to the immediate vicinity of the phone but very quick and easy.

Came across one of these in a Dean Koontz book once but no idea if they exist in real life.

Ah, but with a bug you need to plant it, with this device you just need your victim(target?)'s phone number.

As to the existance of such a device, I’ve heard this before. As KneadToKnow said most phones have power to the microphone physically disconnected when ‘on-hook’. However I’m fairly sure I read somewhere that new phones would respond to a correctly sent ‘off-hook’ signal without someone having to lift the handset. Then someone at the exchange could complete the call to the listeners phone.

So technically it’s possible but certainly not with all phones, if any.

It should be easy enough to check for as your phone will either a) Have an audible dial-tone when hung up, or b) Have no dial-tone when you pick it up.

Even though I work in telecomms most of this is reconstructed from my dodgy memory and may be complete nonsense. Someone will be along shortly to slap me about if this is the case.


I think there was a device, once upon a time, used by phreakers, called an infinity box.

I have used an office system that somewhat inadvertently does this. That system allowed for an “intercom” mode that would connect you to another internal extension with no sound. Obviously you are supposed to start talking, but if someone simply established the connection, they could listen in surreptitiously.

It’s kinda hard to see how something like this could be setup for a user’s home phone without them knowing and without them having special phones designed for such a feature.

When I worked for TPC (The Phone Company) a few years ago, they had a device called an Automatic Line Insulation Tester (ALIT) that could access any line without ringing the phone - this was done for purposes of (as the name implies) testing the line to ensure there was no faulty wiring. As to whether or not the telephone could then be used to eavesdrop on the household, I have no idea - but the technology is there to access the line without ringing the phone, which is half of the question, anyways.

Hmm, I’ve looked around a bit and this site goes on about this stuff.

About halfway down the page. Dunno how reliable that site is.

Searching for ‘Hookswitch Bypass Method’ turns up a bunch of other stuff including people offering to detect bugging of this type.

This page has more info.


SavageNarce thisa might be simular to DATU which is a system that allows you to get tone sent on a line or ‘audio monitor’ or other functions. If you used it to access a line in use you could hear something but any voice is intentionally scrambled. Also you can only hear when the phone is in use

as to the point all you need to do is lift up the phone to check - well no - it could still provide a dialtone if the line was tapped into or the device can supply a phony dialtone and if needed fwd a call you made (or provide a busy).

I doubt such a device exists

A similar device re functionality existed about 20 years ago via (IIRC) BSR and Sharper Image etc. and were essentially all in one home control and monitoring devices so that a user could dialup his house punch in an access code and via X-10 modules control house temp and lighting etc and also “listen in” on his house via a microphone in the unit (sort of a one way (in) answering machine).

I have not seen them marketed lately but they may still be available.

You cannot do this with a regular phone as others have indicated because the microphone cannot be activated without picking it up.

No biggie., I bought an answering machine, like $20, had this feature in it- sort of, it would
let you listen in on the room. I don’t recall if it makes a dial tone.

It was called an infinity box. I’ve never seen one myself, but they’ve been explained to me by phreaker friends.

This is nothing to worry about, because (a) as noted above it requires modification of the spyee’s phone device (including disconnecting the ringer), and (b) the phone network no longer supports it.

The device is based on principles to the famous “black box” which in the days of electro-mechanical switches could allow free (low-quality) phone calls. The idea is that the phone company knows you’ve picked up the phone by a changing resistance on the line. If you could rig the device to let enough current through to drive the speaker/microphone but so small that the phone company didn’t notice, two phones could be connected without the telco knowing that a phone had been answered.

The phone companies really hated this, not only because the caller got a free call, but because through the duration of the call the telco is still wasting energy by sending the ringing signal to the callee.

Today, it doesn’t matter because there is no connection between the phones until the switch sees the callee pick up the phone. Note that this means the ringing sound you hear when you place a call is only synthesized, not an actual artifact of the callee’s phone ringing. Also, after a few rings without answer the switch abandons the attempt to connect.

[slight hijack] Would that have been Dark Rivers of the Heart, maybe?
[/slight hijack]

no, it was Strangers. the one about the, well, strangers, meeting up in like Las Vegas or something, all of them having gone through an horrific experience (i will not spoil the book- haven’t read it in almost a decade, but i remember liking the book a lot- read it!).

anyway, the device is an infinity transmitter. old versions of the infinity transmitter would make the callee’s phone busy, but not so with the newer models.

you would hook the transmitter up to a phone line, phone the number, and voila! the microphone in the hand set of the callee would turn on. as long as the phone was plugged into a jack, the microphone could be turned on. and the caller could listen to his heart’s content.

this machine would not work on cell phones, and i’m not sure about newer phone networks and newer phones. most certainly would not work for cell phones (although i’ll bet there’s an ingenius bit of programming which can turn on the microphone from a distance).

that’s all i can remember,


p.s.- also, strangers talked about a laser-based microphone, which is also a real device. point the laser at a window, and sound waves bouncing off the window make the laser spot vibrate slightly. a camera on the microphone picks up the vibrations of the laser, and translates those movements into sound. so even if the infinity transmitter doesn’t work anymore, you can still be surveilled.

oh, and here’s a few more things about the newer infinity transmitter which some posters have touched upon:

  • as i said, the callee’s phone can still receive calls. and those calls can be listened to.

-also, the callee can make calls. while being surveilled by an infinity transmitter, the telephone still presents a dial tone if you lift up the receiver.

-if, after making a phone call, the callee hangs up the receiver, the infinity line remains open.

‘newer’ is like mid-eighties, by the way. i know nothing about the next generation. and these transmitters are not just the domain of the federal government. you could buy 'em in switzerland, if i recall correctly.


snarked! yet again, i did some research, and found my memory to be shit!

infinity transmitter’s gotta be plugged into the callee’s phone line somewhere, either the phone itself or somewhere else on the wire.

but spacedog’s cite, a bit further down than the IT material, talks about some other devices which i believe i am conflating with the IT.


ok, i don’t know whether it would be possible to eavesdrop using a normal phone in a normal house without bugging it first, since i support the theory that others here have mentioned, that an on-hook phone cannot transmit. Unless the ALIT that SavageNarce refers to could do that, but i seriously doubt it.

But it is possible using the following methods:

  1. get a spy phone available from spy product sellers. This will allow you to call up the phone (which looks like a normal phone) and listen in to the conversations in the room, because the instrument will pick up your call without audibly ringing and allow you to listen in.

  2. This one is a more interesting method. I have tested it (on myself) and it works. Any decent cellular phone has a handsfree option. Within the handsfree menu it has an auto-answer function. If auto-answer is set to Yes, then if you connect a handsfree headset to the cellular phone and call it up, it will automatically pick up your call after the first ring. If you set the ringer to silent, it will pick up your call without making a sound. You can extend this method without the need of a handsfree set. Just short the terminals that the handsfree set connects to, thus fooling the cellular phone into thinking that the handsfree set is connected. “Headset” or something similar will display on your screen, confirming that the cellular phone thinks the headset is connected. Set auto-answer to on, ringer to silent. Now when you call up the cellular phone it will pick up your call automatically and silently and work as a transmitter/receiver thus allowing you to monitor the surrounding location.

Disclaimer: I offer this information not for you to use it incorrectly, illegally or even amorally.

  1. Call Opal, and say: Hi, Opal! I’m listenin’ in…

Also, to answer the OP’s second question, it might be possible to use counter measures for some methods. for example, method 2 above can be detected by one of those neat flashing led thingies that start glowing when a cell within its range starts ringing… it detects radiation in its vicinity and the leds start dancing.

However, the military has at its disposal some amazing spy tech. Most of which you would not have resources to counter. The average Joe could also easily setup video/audio surveillance without your knowledge and with little investment in your own backyard and it would be difficult for you to detect.

I believe the device existed. I think a version came out of the CIA or FBI labs and was used for clandestine surveillance, and did not require phyisical access to the phone. It’s the kind of thing that might have been designed for a particular operation (and a particular type of phone) that proved to have broader, but not universal, applications.

Acoording to Mark Bowden in Killing Pablo (about the covert US hunt for Pablo Escobar), the CIA also has a device that will make cell phones transmit at low power without alerting the user, thus allowing tracking from surveillance aircraft.

So I’d say there’s about a 100% chance that the device you describe exists, and works on SOME telephones.

I believe such a device is descibed in the book “Spycatcher”. It is evidently quite real.