I opened the microphone compartment on the antique phone to see what I was working with. It looks like this phone has been repaired before and the installed microphone probably isn't the original. Based on other repairs/modifications I'm seeing, it looks like this phone was converted to use a modern tone based system at some point.
I 3D printed a bracket to hold the digital I2S microphone in the space where the original went, and then decided to add space for some neopixels on the sides. I think these will be visible through the mouthpiece and be able to provide some basic visual feedback without being able to see them when they're off.
I need 7 wires for the digital microphone plus the neopixels, so I ran a slim cat6 cable through to the mouthpiece. I'll add an RJ45 jack to the enclosure so that this can be easily plugged in or unplugged if i need to remove the computer or other circuits.