When we talk about dumb phones we still have to distinguish between basic phones (just call or text) and feature phones (limited access to the internet and some apps). I believe at the dumb store they mainly serve feature phones.
According to Allison Burtch and Ramsey Nasser, the authors of Dumb Store, “The Dumb Store is a demonstration of what can be done using dumb phones. It’s an exploration in resisting planned obsolescence and using current technology to its fullest capacity.”
In the online store you can get stock prices delivered to your phone, turn it into a flashlight, play rock, paper, scissors, have access to the first three sentences of wikipedia entries, you can tag or view a public wall, get your TV program, quotify, see if the L train is fucked again or receive random Haikus.
Extra resource on dumb phones here.