GSM blocker jamming devices suppress cell phones by sending signals on the same frequency
The mobile phone is designed to increase power when subjected to low-level interference, so the jammer GSM must recognize and match the power increase of the mobile phone. Some jammers only shield one frequency used by the mobile phone, which will have the effect of shielding two frequencies.
Some high-end devices block all frequencies at once, while others can tune to a specific frequency. The mobile phone was tricked into thinking that it had no service because it could only receive one of the frequencies. The power of the signal should be high enough so that the two signals collide and cancel each other.
Less complex devices can only block a group of frequencies, while complex jammers can block several types of networks at the same time to prevent dual mode or the third mock examination telephones that automatically switch between different network types to find open signals.