The FCC (Federal Communication Commission) prohibits use of personal cell phones while inflight. This ruling applies only to commercial carriers. If your company has a private plane then you can use your cell phone while inflight.
The FCC says that safety is the reason for the ban on cell phone usage. They believe that cell phones can interfere with the aircraft instruments. The FAA does support this decision.
It is up to the individual airlines to enforce the no cell phone usage rules.
To prevent problems for yourself while traveling, remember to turn off your cell phone before boarding any airplane. If you are flying an airline which is an American company, you may face harsh penalties for using your cell phone while in-flight. Each airline has their own rules and punishments for getting caught using a cell phone.