Mobile phone signal Detector
I want to know if there is any 'mobile phone signal detector' already available in the market.
The purpose is to find if anybody is using mobile phone for calls or messaging at restricted places. Jammers are not fine as some are exempted from making calls etc..
If there is nothing like that available, how to make one to detect the signal or even the phone from which the signal was emitted.
Usually when we are about to receive a call or sms and if we are near a Speaker we get a specific noise through speaker, or if we are near a monitor (CRT) it flickers. So I am sure there are ways to find this signal emission.
Please suggest the options to do this....