Looking at the hardware, it's likely to be a "mostly usable interface to a cell modem." So, SMS, audio. No idea if it can handle MMS or not. And anything beyond that is iffy.

They've got some interesting concepts for a separate application processor that can route to the screen, but... right now, consider it the sort of project to build if you don't actually need to talk to anyone with any reliability.

Current status section mentions "3d printable phone case designed in FreeCAD", but there don't seem to be corresponding file. I think the project is practically in breadboard stage.

The main processor is a very low-performance microcontroller, so even "featurephone" might be ambitious.