There are no FAA regulations regarding phones/electronic devices other than a blanket rule that you can only enable them with the permission of the captain. The rules regarding cellphone use on planes is from the FCC. They don't want tubes full of transceivers broadcasting at max power (phones ramp up power to search for towers where there are none) high above the earth where they can broadcast over the greatest physical area. At 30k+ft this is not a huge concern (too high for a noticeable increase in the noise floor) but in terminal areas they could conceivably tie up bandwidth on every tower in the city. Whether this is a concern with modern cell towers is up for debate however.