mobile jammer

Can anyone provide any info abt 'mobile jammers' .
They block/jam mobile frequencies and device fails to get service provider's range.
Used in theatres, cinema halls, class rooms, church etc...
How they work, design? cost (in genral, for small distance) ?
regards,
Neil.

try searchin. a similar topic was up a not so long ago

after (very) little searching https://www.electro-tech-online.com/threads/help-with-robot-project.9561/