Bertrand Fan, an engineer at Slack, bought a used payphone on ebay and installed it in his house. The payphone had come from a Las Vegas casino ("which probably explained why it appeared to have personally smoked a pack of cigarettes") so he spent time cleaning it before mounting it on the wall. He also bought one of those neat 1980s transparent Radio Shack corded phones and installed it in his young daughter's room. He connected the two phones with a magic box called a phone line simulator.
This basically creates a closed network between two phones. You can configure it so that if you pick up one phone, the other phone rings, and when that phone picks up, you can talk, and vice-versa. So I bought one, drilled a hole to my daughter's room, and ran another RJ-11 cable to her phone.
I'm not sure exactly what I was expecting, but at random hours in the day I'll be working in my office and the payphone will ring and my daughter will tell me about her dolls for a minute or so and then I'll say, "That's great, but I have to get back to work." and we'll say goodbye until the next time. Most of the time, it's a nice break.
Read his build notes for more details and cute pictures.