The Tech Already Exists
The THOR system in use by US forces is a phone signal jammer to prevent cell phone and radio initiated IED detonations. Basically what it does is detect radio signals and identify the troughs and peaks of the frequency wavelength, it then modulates its output to produce a peak where the detected frequencies trough is, and a trough where the detected signals peak is, only at a LOT higher power output. To explain it with a metaphor, if you were trying to talk to somebody and I stood behind you repeating everything you said only slightly out of sync and at a full scream then you would be unable to speak to anybody. Basically the same concept, the cellular and radio devices are not able to identify and translate the frequency into anything useful through all the noise.
They are quite small, the THOR pictured here is actually the last gen system, the new MJOLNIR system is about 1/3rd the size at the same power output. They have a line of sight limitation as walls and stuff stop them from working, but with no obstructions systems like this have an effective range of several hundred meters.
As far as legal ramifications, they aren't really allowed to be used within the US because of regulations by the FCC. They can potentially interfere with emergency equipment, sensitive electronics, emergency responder radio traffic, and life saving medical devices like pace makers. They pump out a LOT of power, if you wear one for too long you get a splitting head ache from the radio frequencies resonating water molecules inside your body. The larger ones like the CREW system deployed on vehicles can black-out transmissions at up to a kilometer and if you touch the antenna while it is operating like I did on accident once you will get a radiation burn on your hand and all the skin will peel off after a few days. The biggest legal ramifications of such a system in commercial use are liability related. High power High frequency RF transmissions are nothing to mess around with. This is primarily why the FCC regulates which frequencies and power outputs can be used in certain places and for certain applications.