The way it's usually implemented is two sides of a wall have a mesh in them, similar to chicken wire.

Jams all signals, doesn't require a beacon to be constantly transmitting.

I'd assumed "jamming" is when something actively disrupts communication, not passively prohibits the signal from reaching its intended recipient, which sounds more like "shielding".

(Maybe this is unnecessary nitpicking, and everyone else understands that it was informal; English is not my first language.)

Native English speaker: you're correct (ie, i agree with you).

So it's a faraday cage?

Yes, but part of the infrastructure of the building.