A mobile phone blocker jammer emergency calls made to doctors and first responders. For example, a jammer can stop the residents of a house to make a call to the police, when thieves are burgling their house. This is why a jammer is illegal, according to telecommunications regulations in the United States. The FCC (Federal Communications Commission) thinks a jammer interferes with the public airwaves, and a jammer infringes on a person's right to use a cell phone. A jammer also interferes with medical devices, such as pacemakers, used by heart patients. You have to pay penalties, if you're found using a cell phone jammer.

Mobile phone jammers are wanted by government, law enforcement agencies and the military. In Iraq, terrorists use cell phones to trigger explosives. The army can use a jammer to make mobile phones go dead, so that army technicians can deactivate the bomb. There may also be a law that allows officials to install jammers in prisons, to eliminate cell phone use by prison inmates.