Back in 2011, I came across an old, broken TDD (Telephone Device for the Deaf). I asked by friend what I should make with it, and he suggested making it play a text adventure game, like Zork or the Colossal Cave Adventure. I tried porting those to an Arduino Mega, but eventually had to use an old phone, modem and a small computer to make it work.
Flash forward to 2016, and now you can fit enough computing power inside the phone to do away with the TDD, so I did. I found a really cool old rotary phone (mine is from Holland, circa 1950s or 1960s), for $30 on eBay, and gutted it to make room for a CHIP computer, battery and a USB sound dongle.
A more complete write-up is in the build blog.