Going back to the 1890s, there was a series in Boys Own Paper about making (literally here) and using your own internal telephone system. The only ready made part they suggested was the microphone, though the description of how they worked suggested a homebrew version (admittedly using parts possibly not available then) using a carbon rod from a U2 and a couple of dynamo brushes. It worked quite well! It was also, oddly enough, the
only part that I actually made
