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From: lawre...@gandi.cluon.com
Newsgroups: sci.electronics.misc
Subject: Re: car phone
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Date: Sat, 07 Jan 2012 03:37:54 +0100
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Sonnich Jensen <sonnichjen...@gmail.com> writes:
>
> I have been told, that only one (or a few) could use it at a time as
> all phone were on the same frequency.

(Context:  Assuming IMTS service from late 60s through early 80s) 

Depends on the locality -- there were about a dozen channels available
in each of two bands (one around 150 MHz and another around 450).  All
the tech details you could want are in the Wikipedia article:

  http://en.wikipedia.org/wiki/Improved_Mobile_Telephone_Service


> Therefore everyone could listen to the phone calls.
>
> I wonder how they contacted the car.

In any one "system" (i.e. metro area) one of the available channels
would transmit a continuous "idle tone" and all receivers would scan to
that channel.  Signalling came from the tower - all mobile sets would
listen to the signalling spill for their own Mobile ID.  As soon as your
mobile knew it wasn't being signalled it would pick up scanning.  If
there were NO idle channel, it would typically illuminate an "all
channels busy" lamp.

>
> There is this story - that a guy phones his wife, that he will be
> late, then his mistress - for everyone to listen to
>

Everyone who worked in the industry has a story like that - it happened
often enough that it was a common occurance.  Or something equally
devious.

--N1GAK