Mobile Phones do cause interference. My captain also kept his phone on until push back, in case of late message from operations, then would turn it off and drop it in his flight bag. Well you can guess what happened. One flight he dropped it in his bag and promptly put another binder inside his bag turning the phone back on. All throughout the flight we kept hearing the phone searching for a cell tower. Really irritating over the headset in the cockpit. Thank goodness, the captain had to get something later out of the bag and found the phone on.