Hello Mr. Derek Sufi,

Below is a reposting of my above answer but
this time it is done using HTML. This circuit
as potential for all of your needs. Read the
other posts at the internet address and you
will see how to increase the Audio.
Regards,
Charles Bannister
http://www.ee.washington.edu/circuit_a...HEMTEL_010

<h1>Use old phones as an intercom</h1>

From: mwandel@bnr.ca (Markus Wandel)
<p>I have recently thought about this and come up with a kludgy but workable scheme.
<p>Talking over the phones is easy. You put
DC current through the phone and it transmits
and receives audio. So two phones and a urrent
source (about 25mA)all in series will give
you a talking circuit. A suitable current
source an be as simple as a 9V battery and a
series resistor whose value is adjusted
(with both phones offhook) till about 25mA
flows. You can then bypass the battery and
the resistor with a capacitor to couple the
audio straight across and get a loud and clear
connection.

<p>What is much harder is signaling the
other end. To ring the bell you need to put
90V (RMS) 20Hz AC into the phone (nominally).
Lower voltages will work (down to about 40V)
but different frequencies won't. You can't
ring the phone at 60Hz. I have a ringing
circuit in a PBX I built but it consists of a
20Hz sinewave generator, a push-pull power
booster and a big transformer. Much too

elaborate for a simple 2-phone intercom
circuit, and anyway the ringing voltage could
painfully zap a kid.

<p>So forget the bell and look into other
forms of signaling. This is what I have come
up with:

                       +  | | -
+-------+------ - - --+---||||---///--+---- - -----+-------+
|       |             |    | |     R    |            |       |
|       |             | 24V             |            |       |
|      ---            |                 |           ---      |
|     |   |           +---||------------+          |   |     |
|      --- Sonalert       C                Sonalert ---      |
|   C   |                                            |   C   |
+---||--+                                            +--||---+
|      _|_,                                         _|_      |
|      /   15V                               15V    /      |
PHONE    -+- Zener                             Zener `-+-    PHONE
|       |                                            |       |
|       |                                            |       |
+-------+------------------ - - - -------------------+-------+


As before, set R to give you a talking 
current (both phones offhook) of about 25mA. 
Start with 1K ohm. Leave it in if the phones 
work well enough; the current is not very 
critical. The capacitors C are audio bypass 
capacitors and should be about 47uF.

<p>When the phones are onhook they present
an open circuit, and the 24V battery voltage
is not enough to overcome the 30V series drop
of the Zeners and no current flows. When both
phones are offhook they present a very low
resistance and the talking current
(determined by R) flows.

<p>When only one phone is offhook it
places its low DC resistance across the Zener
diode on its side so that the full 24V supply
is applied to the other side. This overcomes
the voltage drop of the other Zener diode so
the other Sonalert beeps. The wonderful thing
about Sonalerts is that they make a loud
noise with only a few milliamps of current so
the series resistor R doesn't matter.
Especially nice is a pulsing Sonalert which
goes "Beep beep beep" automatically.
While the far-end Sonalert is beeping, you
hear the beeping in the near-end receiver (at
low volume thanks to the bypass capacitor
across the far-end Sonalert) to confirm that
the line is working and the other end is being
signaled.

<p>The power supply can be three 9V
batteries in series but since 80% of the
power is lost in series resistor R rather
than in powering the phones it seems a little
wasteful. A 24V wall wart with clean filtering
would be better.

<p>The signaling components can be mounted
inside the phones. Only two wires are needed
to go to each phone, and the power supply can
be mounted centrally, out of harm's way. If R
is adequately big (1/2 watt) and has enough
ventilation then both lines can be ndefinitely
shorted out without any fire hazard and there
is not enough voltage anywhere to hurt anyone.

<p>I have tested this with 500-type phones
and two different types of piezo buzzers
(pulsing sonalerts and non-pulsing brand X
ones) and it works great. You should be able
to get all the needed parts including piezo
buzzers at Radio Shack. I love telephones. Too
bad I don't have any kids who want an
intercom line.</p>