Cellphone jammers are usually these are used to prevent important information from being leaked. Such as people that work in the Pentagon for example will have no access to dial out because of these cellphone jammer.
Jamming a cell phones will indicate that there is no service or no working signal.
Some cellphone jammers actually work in a radius of 1 mile of the disasterous in case of an emergency.
Cell phone jamming is banned in the United States, United Kingdom, Australia and other countries.
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source:
http://newsinfo.inquirer.net/inquirerheadlines/nation/view/20100202-250811/How-cell-phone-jammers-work
