A mobile phone jammer is an instrument used to prevent cellular phones from receiving signals from base stations. When used, the jammer effectively disables cellular phones. These devices 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.

But in some special place, it plays a useful role. for some meeting rooms, conference rooms, museums, galleries, theatres, concert halls, churches, temples, classrooms, training centers, factories, banks and so on.

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.

• Australia: illegal to operate, supply or possess
• Brazil: illegal, but installation in jails have been proposed.
• Canada: illegal, except by federal law-enforcement agencies who have obtained approval
• New Zealand: Illegal to sell, manufacture or use. Legal inside jails by Department of Corrections.
• Singapore: illegal to manufacture, import, use or sell radio jamming equipment other than by or for supply to a permitted person.
• Sweden: illegal. Legal inside jails and for military use.
• Ukraine: legal, planned to be used in schools
• United Kingdom: illegal to use, but legal to own. Having been proposed by prison inspectors, installation and use in jails has been legal since the end of 2012
• United States: Cell phone blocking devices are used by federal officials under certain circumstances. Privacy rights of property owners may affect the policy and application of law within buildings. For radio communications, it is illegal to operate, manufacture, import, or offer for sale, including advertising (Communications Act of 1934). Blocking radio communications in public can carry fines of up to $112,000 and/or imprisonment of up to one year. The Homeland Security Act of 2002 may override the Communications Act of 1934.
• India: illegal with no law to counter the illegal jamming except for Security/Military agencies.Can be used in Jail, Theatres, Mosques, Schools etc with prior permit and jamming strictly limited to the firm perimeter with zero leakage