Well as far as I know you are not allowed. But thankfully it is not enforced, if your phone is not visible nobody is not going to tell you anything. That doesn´t meen that somebody at the cockpit keeps trying to reset a computer with no results. We don´t know phones are the cause, there is no way to know that unless you analyze all the signals of all the data busses and check what is going on. Of course not all the malfunctions are due to the phones, but we have enough of them to tell you that phones have something to do with some specific failures we experience. How much? nobody knows.