[AMRadio] auto-dial


Geoff w5omr at satx.rr.com
Wed Aug 3 07:23:46 EDT 2005


VJB wrote:

>Solution to the random dialout issue depends on
>whether the vulnerable phone actually makes and breaks
>the line. 
>  
>

Not according to some of the other folks around here, Paul.  Dave/W6PSS
suggested that it was probably an electronic device that was full of 
millions
of transistors and what not, (it is) and that enough forward bias at the 
base
of one of those non-protected, RFI prone transistors would be enough to
take the line 'off-hook' - after that, it's just a myriad of complex speech
patterns that cause the phone to either pulse, or create an emulation of 
touch-tone frequencies.  Either of which will have the same effect.

Have you ever listened to a V/UHF repeater that has a touch-tone gaurd
on it?  That's where the DTMF tones are muted before they come through
the output of the repeater.  It helps the control operators to keep 
control of
the repeater instead of making the control codes available to anyone who
has a DTMF decoder.  When the DTMF mute feature is on, what you hear
instead of the tone, is a single tone, repeating 'beep'.  Sounds like 
"T"'s being
sent, at around 15wpm.   Sometimes, the human voice will activate that
gaurd (females especially, due to the higher pitch of their voices) and 
you can
hear that DTMF mute signal come in, and all other audio is blocked, 
until it
drops out (a second or two).

Kick the transmitter on, forward bias the off-hook transistor, pump in 
severely
EQ'ed audio.. dial randomly.  Makese sense to me.

 I wonder if there were any long distance calls made?  ;-)

>Some office I worked at used to lock the dial on the
>rotary phone, but if you clicked the receiver hangup
>bar quickly enough, you could dial your number just
>fine.
>  
>

I've done that before, as well... at a place I used to work.

>If the call remains completed, then just dialup your
>computer service or something else, and keep the phone
>line busy so the clicks cannot obtain a dialtone and
>cause mayhem.
>

Am on cable.  I will, however, take a look into installing some sort of 
filter
on the phone line, at the box.  A 1mH inducktor on each leg and a cap
across 'em, and to ground ought to take out -most- of the RFI that's
plagueing my poor phone line (cheap $9.95 Wal-Mart phones, anyway)

For sure, it's something to look into.

---
73 = Best Regards,
-Geoff/W5OMR





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