Mobile phone jammer
A mobile phone jammer or blocker is a device which deliberately transmits signals on the same radio frequencies as mobile phones, disrupting the communication between the phone and the cell-phone base station, effectively disabling mobile phones within the range of the jammer, preventing them from receiving signals and from transmitting them. Jammers can be used in practically any location, but are found primarily in places where a phone call would be particularly disruptive because silence is expected, such as entertainment venues. Because they disrupt the operations of legitimate mobile phone services, the use of such blocking  devices is illegal in many jurisdictions, especially without a licence. When operational, such devices also block access to emergency services. Legality. Since these jammers actively broadcast radio signals, they may or may not be legal to possess or operate based on the specific laws of the area one is in.