The main purpose of a cell phone jammer is to stop all cell phones in a certain area/location from being able to communicate with the cell phone tower. Although the concept is simple, the methods used to achieve this are quite complicated.

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Cell phone jammers work by targeting frequencies that cell phones typically use to communicate. Cell phones use different frequencies because they send and receive signals simultaneously at all times. Cell phones can also switch between frequencies to try to find the best channel for communication. 

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Selective jammers attempt to block cell phone signals by jamming only one frequency. This one single frequency jamming can trick the device into believing that there is no signal. In turn, the device will not function.