From: Brian D Williams <talon57@well.sf.ca.us>
Date: Mon, 6 Feb 95 12:29:02 PST
To: cypherpunks@toad.com
Subject: RE: Firewall chips
Message-ID: <199502062028.MAA25818@well.sf.ca.us>
MIME-Version: 1.0
Content-Type: text/plain




>> 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).

>Even if you did not buy this feature when you bought your phone,
>it is still there, just waiting for that electronic tone.  You 
>can't produce it, because you didn't buy the doohickey, but anyone
>with such a doohickey can call your house and listen in. . .


> S a n d y

>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 Devices of this type are known generically as "infinity
transmitters." They will not function anywhere the phone network is
run by a digital (4ESS or better) switch.

 In a modern switch, the ringing you hear is being generated by the
switch, the switch is simultaneously ringing the number you dialed,
however the two are not physically connected. The actual connection
is not made untill the far end answers, this was implemented to
stop various types of fraud, and unauthorized eavesdropping.


Brian D Williams
Cypherpatriot