This is another old one from back in college. I stuffed a microcontroller in a rotary phone to act as a stage in a scavenger hunt puzzle. When the phone was picked up, it would start beeping out a question in morse code. The player would need to answer but entering a number with the dial. This would give the next part of the bigger puzzle with some very basic text to speech.
This preceded github but here’s the files: Atmel Studio Code Dump