Not completely true. It is because they have not been tested to ensure that they do not interfere with any aircraft systems. They could possibly interfere. Since they haven't been test, it is assumed they will interfere so they are prohibited during critical phases of flight.The reason that you can't us electronic devices during an approach is not based on any interference issue. It is based on having you unencumbered and able to hear and react to emergency instructions given over the PA system.
There have been reported cases of cell phone interference. Since there is no way to reconstruct these incidents, it is assumed they happened.