For many years, the FAA prevented people from making phone calls during a flight. Was this because a phone call interfered with aircraft communications?
While there is a chance it can interfere with aircraft equipment there are lots of other reasons as well.
It's disturbing, you would not want a can full of people yapping on their phones for 5 hours while others tried to sleep. Lets be honest does asking someone to "Keep It Down" ever really work.
It's very taxing on the cell network itself. Without going into lots of detail about how cell phones work, the short and sweet of it is that when you are moving (say in a car) your phone is constantly hopping form tower to tower. At the speeds a plane moves at you would hop tower at a rate that would be far more taxing on the system than when you are in a car and could lead to dropped calls and other connection issues.
You need to listen up! While the laws on in flight entertainment have become more relaxed over time, one of the reasons you are not to use things during takeoff and landing is these are critical times in the flight (for something to go wrong) and if it does, you need to be attentive and able to listen to instructions. This, on some level, holds true for the whole flight as well I would think.
The use of personal electronic devices (including cell phones) during flight is still generally prohibited by the FAA, unless the operator or PIC allows it. FAR 91.21 the current wording on the use of electronic devices
§91.21 Portable electronic devices. (a) Except as provided in paragraph (b) of this section, no person may operate, nor may any operator or pilot in command of an aircraft allow the operation of, any portable electronic device on any of the following U.S.-registered civil aircraft:
(1) Aircraft operated by a holder of an air carrier operating certificate or an operating certificate; or
(2) Any other aircraft while it is operated under IFR.
(b) Paragraph (a) of this section does not apply to—
(1) Portable voice recorders;
(2) Hearing aids;
(3) Heart pacemakers;
(4) Electric shavers; or
(5) 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.
(c) In the case of an aircraft operated by a holder of an air carrier operating certificate or an operating certificate, the determination required by paragraph (b)(5) of this section shall be made by that operator of the aircraft on which the particular device is to be used. In the case of other aircraft, the determination may be made by the pilot in command or other operator of the aircraft.
Advisory Circular 91-21-1C provides further details on how one may comply with this section and also advises that per FCC regulations part 22.925, the use of cell phones while airborne is prohibited due to the potential for interference with ground-based communications networks (cell phone signals travel much farther from an airborne source vs. one the ground one) UNLESS the aircraft has special equipment installed in order to help reduce signal interference.
The prohibition on making phone calls is the domain of the Federal Communications Commission (FCC), not the FAA (the FAA has blanket bans on portable electronic devices but not the actual use to make phone calls).
The FCC regulation question is 47 CFR 22.925:
§22.925 Prohibition on airborne operation of cellular telephones.
Cellular telephones installed in or carried aboard airplanes, balloons or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off. The following notice must be posted on or near each cellular telephone installed in any aircraft:
“The use of cellular telephones while this aircraft is airborne is prohibited by FCC rules, and the violation of this rule could result in suspension of service and/or a fine. The use of cellular telephones while this aircraft is on the ground is subject to FAA regulations.”
As far as I'm aware the original reasoning for this prohibition of airborne cellular phone usage dates back to the original analog cellular networks in the US. The old analog system had issues when your phone could communicate with many towers at once and this was the case if you put the phone at some altitude above the ground giving clear line of sight to many towers. This caused problems not only for the phone but also the network.
Current digital cell networks do not have these kind of problems but for the most part are unusable at altitude due to design decisions of the network (primary radiation lobes are horizontal and sometimes down tilted relative to the horizon, not pointing up into the sky). There are also issues with speed that make communicating with ground based towers possible (if you can talk to them at all).
An excerpt from the airborne cell NPRM:
6. Part 22 of the Commission’s rules prohibits the airborne use of 800 MHz cellular telephones, including the use of such phones on commercial and private aircraft. This prohibition was adopted in 1991 to guard against the threat of harmful interference from airborne use of cellular phones to terrestrial cellular networks. The Commission’s prohibition was not to ensure interference-free operation of avionics equipment. When the prohibition was adopted, the Commission noted that a cellular telephone used onboard an airborne aircraft would have greater range than a land-based handset, and its signal would be received by multiple terrestrial cell sites in a given market, causing harmful interference. Moreover, the Commission found that because a cellular telephone can transmit on all assigned 800 MHz cellular frequencies, a single handset could interfere with cellular systems in multiple cellular market areas simultaneously. Thus, the Commission concluded that “the need for noninterference in all cellular transmissions outweighs the benefits that would be realized by allowing the public to use cellular service in airborne aircraft.
The FCC is going to make cell phone calls allowed at altitude, but only in conjunction with airborne cell sites that would be located on the plane.