The point of this answer is precisely that there is no FAA regulation on cellphones, which seems to be supported by this Wikipedia article: https://en.wikipedia.org/wiki/Mobile_phones_on_aircraft

    > Contrary to popular misconception, the Federal Aviation Administration (FAA)
    > does not actually prohibit the use of personal electronic devices (including
    > cell phones) on aircraft. Paragraph (b)(5) of 14 CFR 91.21 leaves it up to the
    > airlines to determine if devices can be used in flight, allowing use of "Any
    > other portable electronic device that the operator of the aircraft has
    > determined will not cause interference with the navigation or communication
    > system of the aircraft on which it is to be used."