On: A Simple open-source Phone programmable with Ardui...

[thomascountz]

Though it's not a "cell" phone as-is (I'd argue it is still a "phone"), there isn't much stopping it from becoming one!

GSM modules and supporting Arduino libraries are readily available, e.g. https://www.adafruit.com/category/281

Edit: I just saw there's a LoRA daughter board developed, so a GSM board seems not too far fetched! See: https://www.wiphone.io/docs/LoRa/latest/