That’s the AT&T Long Lines building, it is just a building full of servers and phone lines.
The NSA have an office there and would tap lines they deemed suspicious since most calls across the country were routed through it.
And now it’s suspected of housing their mega-computers for illegal mass surveillance. The building is supposedly built to withstand nuclear attack. It’s a building built for machines, and an example of brutalist architecture.
Here’s the documentary
— https://en.wikipedia.org/wiki/Person_of_Interest_(TV_series)