How to Build a Mobile Signal Detector Circuit for GSM Frequencies in Classroom

Hi,I am a student of electronics..I want to build a mobile signal detector for my classroom..When any one in the class is using their mobile phone for calling or texting my circuit should detect that and a buzzer should be on..I have seen many cirucits in the google,but i did not understand the exact working circuit,i.e

1) I did not understand how a particular frequency can be detected and

2)what is the frequency used by our mobile phones.

Please some one help me to do this circuit...

It is not as simple as detecting transmissions on a set of frequencies. Cell phones transmit housekeeping messages to the cell towers at regular intervals even when they are not being used. Determining the difference between these housekeeping packets and text messages would be an extremely difficult project even if it is possible given the encryption used by the cellular networks.

Frequencies vary by country, carrier and location. You can see a list of US frequencies in use plus links for other countries here: https://en.wikipedia.org/wiki/Cellular_frequencies