

Incredibly, and not at all subtle. It’s also of fairly limited effectiveness in most scenarios, wireless signals are generally a lot more complex than what a simple jammer will cover. On top of the difficulty in transmitting a reasonably large amount of radio power in any useful frequency, you have to also jam side frequencies to avoid fail over and certain noise mitigation techniques.
Honestly it’s not even just that it’s massively illegal, it’s just so wildly impractical. Like, what would you accomplish? Radio waves fall off pretty quickly in strength, so your jammer is going to have a limited range, and if it doesn’t you just knock out flight communications and emergency response while cellular hops to one of the other hundreds of frequencies and like 10 modulation protocols until something works, and it’s going to be insanely difficult to jam all of those at once.
Just jamming 700mhz would involve an antenna array that would be bigger than a person and using the old “moar power” approach sees wattage requirements shoot into megawatt ranges pretty quick.
Look up a Faraday cage, much easier to block the signals than it is to jam them.