If those are truly old fashioned phones, (e.g., like 1950's phones) there is probably a way to interconnect things without needing any active elements. After all, at the time when they were designed (1920's?), there were no active elements that would fit inside the phone case and which also would not require bulky power supplies. Basically, the whole phone was passive elements and ran off batteries in the local office, the voltages being surprisingly low. I don't know the exact levels off the top of my head (google is your friend here) but want to say that while off-the-hook, the voltage is somewhere around 7 volts.
The only significant part you find inside an old phone, other than the mic and the speaker, (and disregarding the mechanical ringer and dialer) is some kind of a multi-tap transformer. Not sure what the transformer accomplished. The microphones used a finely divided carbon powder, under a diaphragm, and were basically variable power resistors that respond to sound. I know, from fooling around with them as a kid, that you can put a 6V lantern battery, a carbon mic, and an 8 ohm speaker in series, and you can get a signal through. If you put two such microphones in series with two speakers, and of course, the battery, you should get at least some signal back and forth. The transformer in the phone was (speculation!) probably there to get a better signal into the ear piece by doing an impedance match.
At some point (1960's maybe, or 70's?), they stopped using those carbon microphone elements in favor of more modern microphone technology, at which point there would have to have been an amplifier involved. The carbon mics I've referred to can be recognized by the fact that when you unscrewed the mouthpiece cover, this nearly 2 inch diameter mic element would fall out in your hand. If your old phones are newer than that, just disregard everything I said.
