I've seen it with my own eyes. Flight attendants using PEDs (smartphones) during taxi in and taxi out. Many aircraft have rear galleys where crew are stationed. Out of view from the cabin, but NOT totally hidden. Anyone who denies that this happens does is badly mistaken. I'm not saying all crew do it, but some do. Fact.
Don't know about you lot but where I come from, doors are in the galley, crew are next to doors, therefore, in the rear galley. EZ/FR, BA 32S fleets, AA/UA (pretty much every 737 CONFIG).