I wonder how he found out.
I would guess that there are security cameras just about everywhere in that tower’s machanical/electrical rooms that may have recorded nefarious activities not discovered until the tapes had been reviewed by a professional spookbuster.
The Gov't/NSA doesn't need to physically go into any building to tap the phones. They're directly tied into the phone-carriers (wire, fiber & cellular), and a FISA warrant results in them being able to gather what they want directly from these carriers. BTW - this would include all data traffic as well, whether via the internet or "private" circuits. Big brother stuff ...