If you're talking about ordinary cell phones, it's illegal to use them on US aircraft. The rule was passed by the FCC to prevent airborne users from jamming the cellular networks. Ground-based cell phones work by being within the line of sight of one or more towers. Line of sight is dictated by elevation and terrain. Each cellular tower is capable of servicing a fixed number of users. An airborne user could be within the line of sight of dozens of towers (perhaps hundreds from airliner altitudes), reducing the overall capacity. According to some articles I've read, the fine for using a terrestrial cell phone on a plane is either $10,000 or $20,000.

There's also the concern that the cell phone emissions might interfere with the plane's avionics. That's why those phones you see on planes are specially designed, both to avoid interference with the plane's avionics and to not use the ordinary cellular network.