Does employee use of DOL ‘timesheet’ app mean we can’t ban cellphones?

Q. To curb wasted time and keep workers on task, we are banning cellphones at work. Now one of our employees has told us that she has the Department of Labor’s wage-and-hour app on her iPhone. She says she uses it to track her hours worked, and that we are illegally preventing her from using it. Will our ban hold up legally?