Not hard at all, if you add a 48V DC power supply ($20 on Amazon) in series with the wires anywhere in the house, you will be able to pick up any phone and talk

Not hard at all, if you add a 48V DC power supply ($20 on Amazon) in series with the wires anywhere in the house, you will be able to pick up any phone and talk to the others. For ringing apparently you need 60-100V AC, I'm not sure what a simple solution for that would look like. https://en.wikipedia.org/wiki/Ringing_(telephony) https://en.wikipedia.org/wiki/Ringing_(telephony)