There such a device,
Full details are:
Securicor Cellphone Detector can detect use of a mobile or SRBR within a pre-set range and will give out a pre-recorded announcement to switch of all padio equipment immediately.
* Detection range adjustable from 2m to 30m
* Emits an audible warning
* 12 months battery life
* Dims: (w)200 x (h) 112 x (d) 50mm
Cost is £149.95 plus VAT
Part Code: 17997
Can be bought from a company in Manchester called Nimans Ltd
Tele 0870 444 3101
Technical Support No. 0870 444 3104