The instruction from theatres is to turn OFF mobile phones, not put them on silent, not be discreet, but turn them OFF.
Can I add my vote to the anti-mobile lobby? There are many jobs where you may not use a mobile phone. They are banned in the chambers of both Houses of Parliament and were yours to ring in a recording session you'd be very lucky to be asked back. Common courtesy and a sense of what public performances of plays and music should involve are just the starting points for why phones shouldn't be used in theatres.