British Version of the telephone Black Box - Non-BT approved!
The Circuit:

X------+-----S1-----+------Y
       |            |         S1 - SPST Toggle
       +-----D1-----+         D1 - 50-200volt diode (IN400? range)
       |            |         D2 - Standard LED
       +--D2----R1--+         R1 - 6.1k Resistor, or combination.
       |            |         C1 - 1uf Non-polarised capacitor.
       +-----C1-----+

Note:- the two diodes are placed the opposite way to each other.
Modern phones have four wires going into them. These are normally coloured
red, white, green, and blue. The red and white wires carry the actual line.
The green wire prevents 'bell tinkle', and the blue wire isn't used.
Three of these wires (including the blue and green) pass straight through
the box, untouched. The remaining wire, either red or white - either will
do - should have the box circuit connected in series with it at points
X and Y.

The Theory.
When a phone is answered, approx. 100mA is drawn from the line which tells
the exchange that you have answered. However, only about 80mA is actually
required to polarise the mike element, so by limiting the current, you can
talk to the caller whilst their exchange thinks that the phone is still
ringing. (And thus THEY are not charged.)
The main current limiting is carried out by R1. 6.1k resistor are hard to
come by, so that resistance must be made up with a combination of one or more
resistors. All other components merely improve running:C1 ensures that speech
is unaffected, and D2 is used to indicate that the box is switched on.

The Practice.
Wait for the incoming call to ring, then switch the box on. The LED (D2)
should pulse with each ring. If it doesn't, reverse the polarity of both D1
and D2. Lift the handset and the LED should light continuously - you may
now talk to your caller completely free of charge. To commence charging
halfway through a call, switch the box off - you will hear a click as the
caller's meter (in their exchange) starts turning. Once you have started
charging, you cannot stop it even if you turn the box back on.

The Snags.
As well as being able to work from any normal telephone, the box will work
when the caller phones from a cardphone or old style payphone. However, the
new style coin payphones do not cut in the microphone until the called party
answers. On trunk or international calls, the call time is limited to three
minutes, as the exchange gives up after that time. (It assumes no-one is in
and needn't hold up the line.)
