Simple really
I believe that it is as simple as this. Passengers must not ignore a lawful command given by the Commander, crew or signage!
If I see someone using there mobile phone I ask them to switch it off and stand there till they have, I have had a similar situation as the first one as in someone seemed to switch the phone off but hadn't, lucky for me I was only 5 rows away when it started ringing. So walked back, politely interrupted the phone call and informed the passenger that the phone had to switched off now. After he had I asked to look at the phone's screen and confirmed it myself. The phones I hate at the moment are the ones with an integral MP3 player.
Also on the point of airlines that are having the "new" system of allowing mobile phone use on board and in flight! Forgive me if I am wrong but aren't they fitting an internal receiver for the mobiles, linked to an external ariel. The avionics bay being shielded from this so the signal shouldn't go any further than the passenger cabin?