There was a device on the market a couple of years ago which was basically a mobile phone alarm. You screwed it to a wall - near to entrances and close to critical areas - and when it detected a signal from a mobile it gave out a loud spoken message along the lines of "please switch off your mobile phone" ! We didn't install any because people were concerned over the noise level in sensitive areas like CCU, but I thought they would be useful at the entrances.
I'm not sure if I kept the information, but I'll try and find it and post the details on the site.
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Today is the day you worried about yesterday - and all is well !