Xref: utzoo sci.electronics:13379 alt.folklore.urban:3133
Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!emory!rsiatl!larry
From: larry@rsiatl.UUCP (Larry Kahhan)
Newsgroups: sci.electronics,alt.folklore.urban
Subject: Re: free calls?
Message-ID: <3497@rsiatl.UUCP>
Date: 6 Aug 90 21:55:12 GMT
References: <32457@cup.portal.com> <1990Aug6.124516.8051@jarvis.csri.toronto.edu>
Reply-To: larry@rsiatl.UUCP (Larry Kahhan)
Followup-To: sci.electronics
Distribution: na
Organization: Radiation Systems, Inc. (a thinktank, motorcycle, car and gun works facility)
Lines: 34

In article <1990Aug6.124516.8051@jarvis.csri.toronto.edu> sheasby@dgp.toronto.edu (Michael C. Sheasby) writes:
>
>The other day I was in a mall and noticed a few yahoos gathered
>around a pay phone... they looked around for cops and then
>unscrewed the receiver on the phone (the ear end, not the mouth
>end).
>
>They took out the small speaker and touched the two wires
>leading to it to the handset holder (the thing you put the
>phone back on when you finish the call). Then they dialed
>and quickly screwed the receiver cap back on. Apparently this
>saved them a quarter.
>

There are two basic type of pay telephones in existance; coin first and
dial tone first. In dial tone first type telephonesa loop start interface
is used. In coin first type telebphones, the circuit is a ground start.
The ground start type phone is probably what they were using.

At any rate, in this type circuit, the TIP lead coming from the central
office is open, and the ring lead has -48V (current limited, usually 
through a resistive feed). Normally, the ring lead is grounded when you
insert your quarter, the central office detects current flow, and then
gives you a TIP ground, which allows you to complete the loop with
the receiver off-hook, ultimately giving you dial tone.

What these guys were doing was externally applying RING GROUND to the
telephone loop, bypassing the coin mechanism. I wouldn't recommend
making phone calls in this manner, as phone companies tend to get upset
over theft of service issues. It's not worth it to try to save a quarter
more or less on a telephone call.


Larry Kahhan - NRA, NRA-ILA, CSG, GSSA , & GOA