While flight attendants and intercom announcements do ask that cell phones and other electronic equipment be turned off there are no meaningful checks to see if this was actually done. That alone tells me how much an issue it is: it's pretty much an honor system. Could you imagine if that was how airport security was handled? "You don't have any knives, weapons or explosive devises do you? Excellent, please proceed."
And last I checked cell phone use on airborne aircraft was prohibited in the US by the FCC not FAA--I believe because traveling hundreds of miles per hour over cell networks was not something those systems were designed to handle.