A mobile phone jammer is a jamming device designed to block cellphone transmissions by disrupting the signal of a nearby cellphone tower. In the beginning, Signal jamming devices are used by police and the military to limit or disrupt communications. These portable jammers are available to allow their owners to prevent others within a certain radius from communicating via cell phone. Similar equipment is made to block signals in places where cell phone usage might not be desired, like theaters, churches, and operating rooms.