Given that aircraft fly around in a veritable EM soup (AM, FM, VHF transmission towers, the spark gaps of an angry god, etc.), I would hope that every phone on the plane draining its battery in a coordinated RF scream would be a survivable event. Whether all the chatter raises the noise floor or introduces errors into sensitive measurements is a subtler but more likely issue.
What is outside the airplane is the least of the problems. A large commercial plane has racks of electronic equipment, dozens of radios, weather radar, flight displays, in-flight entertainment systems, power generation and distribution systems, pumps, servos,...etc.
All of those are potential sources of EMI that need to work together as a system. The only reason a cell phone is considered 'risky' is because it un-tested. There is nothing unique about cell phone electronics from an avionics point of view. Similar, and more powerful, systems are already integrated into the airframe.