I am not so sure there needed to be actual physical devices in the tower itself.
“I am not so sure there needed to be actual physical devices in the tower itself.”
For a land line there would. Especially since a lot of business calls are actually on the internet using software like Sykpe only more secure.
I know from working on a similar problem in of all places the US Embassy in the Soviet Union, there are means to penetrate glass windows with a carrier beam that will return conversations in the room.
The problem was to provide glass that would reflect the beam and also be architecturally acceptable. The issue became moot when it was discovered that bugs had been planted within the building, in the steel as I recall. The glass was that originally selected by the architect, solar gray.