Using a Teensy 4.0 microcontroller and high-quality audio shield components, I engineered a fully functional audio guestbook housed inside a repurposed rotary telephone. I followed the excellent walkthrough from this guide, adapting it for improved durability and simplified setup. When guests lifted the receiver, the system automatically began recording their message, which was then saved as a numbered WAV file to an SD card. I customized the Arduino code to enhance file handling and added clear labeling for post-event file retrieval. Careful soldering and component integration ensured reliable function throughout the event.