I’ve gutted the rotary chest-phone that I am going to make into the third gruephone. What’s amazing to me is how little there actually is to them:
Other than the bell, it’s just a dozen or so discrete components on a simple circuit board, mostly capacitors and resistors, with one transformer thrown in. The rotary mechanism is nifty bit of kit, but I didn’t cut that out, since I’m planning on using it (and it’s an integral part of the phone’s look.)