Yet another use for the 9 volt phone cable (pictured here) is to make a very simple intercom system. You won't be able to create a ring sound with the setup detailed here, but buzzers could be rigged up on a separate set of wires. Anyway, here is one more poor drawing of a phone-based creation.

You won't be able to make sound (i.e. make tones by pressing buttons or hear your own breath or voice on the line) until both phones are picked up. At that point, the circuit will be complete. I have seen designs for this that use a resistor, but I have found that to be unnecessary.

Courtesy of eric odegaard

7-16-2005

page 1 (modem to modem networking)

page 2 (acoustic coupler instructions)

page 3 (phone intercom)