I'm surprised that you could get a modem pair to connect with a plain wire. An old dial telephone (e.g. a 500 set) was an entirely passive device, relying on power from the central office end of the wire. The carbon mic didn't need an amplifier - it's just a resistor, with resistance varying according to sound pressure; a transformer subtracted out the mic from the combined signal and sent the difference to the speaker.<p>I did a quick search on 1670 schematics and came up with very little. Note however that several sources identify the USR101 chip as a ROM - that's clearly not the case, as a schematic shows no address or data bus, and you can see a line from the coupling transformer to an RC network and several pins on the device. It's almost certainly the analog front end and the modulator/demodulator.<p>If you could find a datasheet for that device you'd probably be able to figure out how the whole thing works, but I'm guessing those were never available to the public and have been lost to the mists of time.