(I'm a CFI.) 91.21 is irrelevant. The FCC specifically forbids the use of cell phones aboard aircraft, and the FAA doesn't have the power to overrule the FCC in

(I'm a CFI.) 91.21 is irrelevant. The FCC specifically forbids the use of cell phones aboard aircraft, and the FAA doesn't have the power to overrule the FCC in matters of radio interference. The FAA has no rule, and cannot make a rule, that in any way allows the operation of cell phones aboard aircraft as long as the FCC forbids it.