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In article <3685@tank.uchicago.edu>, barry@arthur.uchicago.edu writes:
> A friend recently showed me a "technique" for making free phone
> calls from a payphone.
> 
> I don't want to promote this method---I just want to know what 
> how the phone hardware/software allows this to occur.
> 
> step (1): get paper clip, and straighten it out.
> step (2): puncture the metal cover that is underneath the 
>           part of the phone you speak into---specifically, poke
>           it through the hole closest to where the cord connects
>           to the receiver.
> step (3): insert paper clip into the puncture, and scratch the exposed
>           end against a metal surface while dialing your number.
> 
> [Note: I have not been able to do this succesfully myself---but a random
> sampling of three payphones in the U of Chicago Science Library found
> that all three had the tell-tale hole of step (2).]
> 
> So---How does it work? (And does it work?)

	It is indeed possible for this to work under most circumstances
where: (1) coin telephone is a "traditional" 1C2 or equivalent single-slot
coin telephone set arranged for dial tone first (DTF) service; and (2) the
central office (CO) apparatus is AT&T or Northern Telcom.

	In a DTF coin telephone the dial and talk circuit is enabled upon
picking up the handset, without requiring any coin deposit.  The user can
proceed to dial a number forthwith.  If the dialed number is a "free" call
(such as operator, directory assistance, 911 or 800-number), the call can
proceed with no coin.  If the dialed number is a local call requiring a coin
deposit, the user may be allowed to complete dialing of the entire number.
The CO apparatus identifies the dialed prefix and verifies that it is a
local call; it then tests for an "initial rate" coin deposit.  If the
initial rate deposit is made, the call completes; if there is no money
or insufficient money, the call is routed to an intercept recorder which
informs the user to hang up, deposit the initial rate, and dial again.

	The CO apparatus detects the deposit of the initial rate by means
of a ground placed on the line by the coin telephone circuit.  This ground
is balanced between tip and ring, so that no hum (caused by longitudinal
imbalance) is introduced into the talk and DTMF dialing circuit.

	The coin telephone contains a "totalizer" circuit, which is an
electromechanical device (aided by some solid-state circuitry and relays)
that counts the value of the deposited coins, and provides a contact
closure when the initial rate is achieved.

	The above fraudulent call technique works by spoofing the ground
placed on the line by the totalizer circutry in the coin telephone.  While
shorting one lead of the carbon transmitter to ground does not produce a
balanced ground (thereby resulting in hum on the line), it will usually
result in a condition which will permit DTMF dialing.  Depending upon
the particular CO apparatus, the ground needs to be present as soon as
the first three digits of the call are dialed (if pre-translation is used),
or within milliseconds of the last digit being dialed (if pre-translation
is not used).

	Needless to say, the above technique results in damage to the
carbon transmitter, and is therefore unlawful not only from the standpoint
of the fraud itself, but from the standpoint of causing damage to the
coin telephone.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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