Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!ooc.uva.nl!ropg
From: ropg@ooc.uva.nl (Rop Gonggrijp)
Newsgroups: sci.electronics
Subject: Re: Telephone device
Message-ID: <16484@slice.ooc.uva.nl>
Date: 3 Dec 90 13:48:32 GMT
References: <6806@uceng.UC.EDU>
Organization: Hack-Tic
Lines: 40

schriste@uceng.UC.EDU (Steven V. Christensen) writes:

>	Last month (I think) I saw a project in one of the Radio
>Electronic type magazines about a self conrained telephone line
>controller. Basically it hooked in parallel with all your phones, and
>could be made to prevent numbers beginning with certain prefixes from
>being dialed.

>	Does anyone know how this is done, since the device is in
>parallel with all other phones? How can it prevent me from completing
>calls beginning with "1900..."?

A device that does this needs at least a way of detecting dial pulses (peaks
in line voltage if hooked up in parallel. How would a device that is hooked
up in parallel break the connection though? It cannot hang up, it can only
short the wires so that there is no audio. Needless to say this can still mean
very high phone bills. In this simplest version DTMF-dialling would also defeat
it. Even if it had a DTMF-decoder fitted, you could still fool it by blowing
either:

-so low that the phone switch hears it but the device doesn't (at least the
first few digits

or, if the device is more sensitive than the switch (which is more likely)

-so low that the device does hear you, but the switch still provides a dial-
tone. Then dial a few innocent (local) digits and then tell the CO to call
Mars.....

Conclusion: the only hack-proof way of doing this is by using a device that
detects dialling on one end and repeats it to the CO. This way there is the
absolute certainty no digits are slipped in-between. All other ways lead to
fraud if exposed to the general public (this does NOT mean, the 'safer' method
is hack-proof in every implementation (see COCOTS)).

-- 
Rop Gonggrijp (ropg@ooc.uva.nl) is also editor of  Hack-Tic (hack/phreak mag.)
quote: "We don't care about freedom of the mind, | Postbus 22953    (in DUTCH)
        freedom of signature will do just fine"  | 1100 DL  AMSTERDAM
Any opinions in this posting are wasted on you   | tel: +31 20 6001480


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