I think that Arduboy community might find this interesting.
NokiX – open-source SDK for classic DCT-3 Nokia phones (this series includes, for instance, the famous “indestructible” 3310 model): NokiX - Arkadiusz Wahlig
Video about flashing a new firmware with additional games: Vintage Firmware Modding Nokia DCT3 Phones - YouTube
Video where you can see some more homebrew games: Awesome Nokia 3310 MOD - YouTube
Hmm… I wonder how difficult in theory would be to port Gamebuino Classic games (that console was actually using a Nokia 5110 LCD) to these old phones.