Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!pasteur!helios.ee.lbl.gov!ucsd!brian
From: brian@ucsd.EDU (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: How can I restrict outgoing phone calls?
Message-ID: <1780@ucsd.EDU>
Date: 23 Jun 89 16:43:30 GMT
References: <43afec9f.16321@apollo.COM> <25535@agate.BERKELEY.EDU> <3237@kitty.UUCP>
Reply-To: brian@ucsd.edu (Brian Kantor)
Organization: The Avant-Garde of the Now, Ltd.
Lines: 19

In applications where there were mixed classes of service on a
small keyset system, ThePhoneCo would occasionally use a combination
of reversed polarity and polarity-guard widgets to allow some phones
to dial and other not.  In particular, a friend had an office with
attached living quarters wherein he wanted to be able to have his
residence phone as one of the line appearances on his 5-line keyset
phones.  TPC installed the residence line with tip and ring reversed
so that it wasn't possible to dial out on the flat-rate residence
line from the business touchtone phones (which would have avoided
paying for business message units).  The keyset inside the house
had a polarity guard in it so that it could dial on any of the
lines.

Polarity guards are also one of the main contributors of interference
to telephone lines from ham radio signals.  In several cases the
amount of RF induced into the neighbor's inside wiring was sufficiently
small that only the phone with the polarity guard bridge rectifier
had problems.  Removing it cured the problem.
	- Brian