[sonicanatidae]

A true Faraday cage would kill the signals. I suspect that wasn't designed as a Faraday cage. It's an elevator.

There could also be a cellular extender inside the building. I happen to work in a building with LOTS of materials that block signals from the outside. The windows are bulletproof, the walls contain metal plates, etc.

We have to use a cellular extender to get cell signal inside, however, the building wasn't designed as a Faraday cage. It just happens to hamper signals in a similar way.