The "no cell phones" signs at court entrances refer to making voice calls and/or allowing the phone to produce sound (ringers, alerts, music, etc.) audible to others. You may bring and use mobile devices for other functions, including listening to music via ear buds/phones, as long as this does not affect your ability to hear and converse with opponents and officials. (Of course you may not use such devices to access tactical or coaching information while you are in a match.)