On Sun, 19 Nov 2000, Neil Cherry wrote:
> You know I am becoming dain bread, I work on VoIP and I completely forgot
> about SS7.
Do you design in-band signalling features into your systems to allow one
to seize a trunk with a 2600HZ tone? :)
> > What are you talking about? Payphones have always been ground-start.
>
> I thought these were changed out, I don't get to play much with pay phones.
Well, it depends on what payphones you're talking about, but the Bell
phones have a ground-start line. The private payphones (a.k.a. COCOTs)
generally use a standard business loop-start line, or in some cases a
specially provisioned COPT (Coin Operated Pay Telephone) line that has
certain features useful for billing.
But anyway, the provisioning of the line has nothing to do with being able
to "phreak" (in a blue box sense) from a payphone. In the old days, when
you had to put in a coin before you got dialtone, you used to be able to
steal a dialtone by grounding the phone (like the trick David Lightman
used in _War Games_). With the old phones, the coin caused the line to be
grounded, and since the trunk was ground-start, it would signal the
central office to return dialtone. So you had to pay even for information
and operator calls, although the operator could return your coin (if you
didn't have a coin to begin with you were screwed :) Since it was assumed
the only way the line would be grounded was by inserting a coin, it was
considered secure.
The Bells then changed their phones out to ones that had dialtone first
that you couldn't fool in this manner, but there was still a way to fool
them into making free local calls by grounding the line while dialing.
A very simplified discussion, and of course pertaining to US payphones,
but the basics are there.
Sellam Ismail Vintage Computer Festival
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International Man of Intrigue and Danger
http://www.vintage.org
Received on Sun Nov 19 2000 - 13:28:41 GMT