What is the principle of the instrument for detecting mobile phones? Metal detection? Signal?

At present, most mobile phone shielding devices are used in the examination room. Its principle is to interfere with the mobile phone signal with electromagnetic waves with the same frequency as the mobile phone signal, so that the mobile phone can not receive the signal of the mobile phone base station, thus making the mobile phone unable to be used normally. I don't think the mobile phone detector you mentioned will be used in the examination room, because it can only detect whether anyone is using a mobile phone and who is using it. It can't be detected unless every candidate is equipped with one, or the invigilator walks around the examination room with this instrument. This is unrealistic. The principle of this detector is based on electromagnetic wave induction, which is the same as the electronic dog in the car. The mobile phone will regularly send signals to the nearby mobile phone base station. It means that my mobile phone is on and exists, and the mobile phone base station will reply that the channel is ready for you. You can use the mobile phone at any time. The mobile phone detector detects whether there is a mobile phone by sensing whether there is a signal with this frequency and the principle rate of the metal detector nearby.