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The following info comes from a leaflet entitled 'FREEFONE':
"British Telecoms recent record profits and continuing appalling
service have prompted the circulation of this information. It
comprises a method of making telephone calls free of charge."
DIAGRAM: PARTS REQUIRED:
O----o-------o/ o------o----O S1 = SPST Switch
L ! S1 ! T
I o--------|<--------o O D1 = 1N4004
N ! D1 !
E o--XXXX------>|----o T D2 = L.E.D. (TIL209 etc)
! R1 D2 ! E
I o--------||--------o L C1 = 1uF, 250vac non elec
N C1 E
O----------------------------O R1 = 2K8 resistor
OPERATION:
This circuit inhibits charging for incoming calls only. When a
phone is answered, there is normally approximately 100ma DC loop
current, but only 8ma or so is nessecary to polarise the mic in
the handset. Drawing only this small amount of current is
sufficient to fool BT's ancient 'Electronic Meccano'.
Its extremely simple. When ringing, the polarity of the line
reverses so D1 effectively answers the call when the handset is
lifted. When the call is established, the line polarity reverts
and R1 limits the loop current while D2 acts as an indicator to
show when the circuit is in use. C1 ensures that speech is
unaffected. S1 returns the telephone to normal when closed.
With the phone on the hook this circuit is undetectable at the
exchange and/or on the line. The switch should be closed to make
outgoing calls and when receiving calls from the operator.
The method illustrated here uses the same technology that BT use
on their 0800 exchange to obtain free incoming calls. (Try this
number as a demo: 0800 800 800.)
*****DO NOT DIVULGE YOUR SOURCE OF THIS INFORMATION*****