It's simple to throw together a set of field telephones from 2 common
type American telephones (complete telephones). Simply connect the red
and green wires with at least a 9 volt battery (up to 48 volts DC) in a
big loop, but you'll have to play around with finding and adding the
right value resistor to the loop to limit the volume. DC volts has a
limited range tho. It's very complicated however, making a phone ring,
so for your ringer you can salvage a couple door bells (or other DC volt
device) to act as your ringer with the switch for each at the opposing
end. You can pick up an ordinary phone at some dollar stores for about
$7 each. Since you don't have to modify the phone itself, you can easily
return it to common use.