This is the Arduino based rotary mobile phone built by Redditor Mr_Volt. It has a small OLED display, is capable of making rotary phone calls, and has enough memory to store a single number. Wow! It also has a sleep mode and -- AND -- an FM radio. Your grandpa is so jealous right now. Well, I mean he would be if he were awake to see this and not snoring in his La-Z-Boy. Let's hide his teeth so he calls us whippersnappers!
Keep going for a video of the phone in use.
Thanks to Dougie, who wants a full-size rotary cellphone so people will know he's the most eccentric person in the room. Heck yeah, you should get a monocle too.