I guess the question is, what would happen if everybody actually listened to RMS and stopped using cell phones?
To answer seriously, however, the largest effect of reduced cell phone use would be that automobiles would be much safer, for their passengers and for everyone else.
Listening to RMS would mean that if you do carry a mobile device, that it would use entirely free software including all drivers and bootloaders, that it only communicates with other hardware running entirely free software (i.e. only basetowers or wifi routers running free software), and that all the internet services accessed ran entirely free software (e.g. private email servers instead of gmail, pump.io & GNU social instead of twitter & facebook, mediagoblin instead of youtube, wikipedia, yacy or some other free software for search, etc.