In Greg Egan's[1] latest novel, "Zendegi", the uprising people use cellular handsets reprogrammed into a peer-to-peer mesh network to continue email, messaging,

In Greg Egan's[1] latest novel, "Zendegi", the uprising people use cellular handsets reprogrammed into a peer-to-peer mesh network to continue email, messaging, and phone calls when the authorities shut down the cellular towers. The hardware is already deployed in people's pockets and purses. It just takes a software download to make it happen.[2] [1] Greg Egan is your man if you want to read a sci-fi book about, say, "given that much of physics was derived by viewing the sun, moon, and stars, how would a civilization without any knowledge of an external universe develop physics?" [2] You do have to take full control of the phone's radio, which is non-trivial. They try to prevent that.