Normal telephone wire has two pairs of wire in it, the green and red
pair, and the yellow and black pair. A standard one-line telephone
actually only uses one pair, usually the green and red pair. The other
pair is a spare and can be used if a second phone line is added to the
house, or for other purposes. On standard four-pin connectors, the
green and red wires use the middle two pins (2 and 3) while the yellow
and black wires use the outer pins (1 and 4).
Telephone's don't work by having one wire go to the earpiece and one
to the microphone, but instead the wires form a circuit, with both
wires going to both pieces. The websites I've linked to below explain
just how they work much better than I could, so I'll leave you with
I hope this answers your question! If you need any further
clarification, please feel free to ask before rating this answer.
Phone-man's Home Phone Wiring Advice Page
Howstuffworks "How Telephones Work"
Google search terms used:
Clarification of Answer by
26 Jan 2003 19:51 PST
The information I posted the first time around wasn't quite accurate.
In the simplest possible telephone you could make, it would just be
the two wires inside the phone, but obviously most telephones around
these days are more advanced than that. So I dug a bit deeper, and the
good news is, in a handset the microphone and the earpiece are indeed
connected separately. The microphone uses the green and red inner
wires while the earpiece uses the outer wires. There's a nice picture
of this on the wiretapping page I've included below. This should make
what you want to do quite straightforward since, as you mentioned in
your clarification below, you can just cut off the microphone circuit.
I wanted to be absolutely sure this time, so I grabbed an old phone
handset that was sitting around unused and pulled it apart. It looked
pretty much like the handset on the wiretapping page below (well
except that mine wasn't bugged =)). In my search I also stumbled
across a very extensive telephone page which was useful in finding
your answer, and which you might be interested in.
Telephone technology page
"How Wiretapping works"