From what I hear, an agency can apply for a broad FISA warrant, for the Trump Tower for example, to listen for foreign influence in the elections (for example) and then (legally) wiretap every swinging peewee in the Tower.
They never go into the building. All of the interceptions are done at a Central Office. The take everything coming out of that area and sort it out later.
That, under the loosened, broadened criteria the 0bamanation rammed through just before the Trump inauguration, likely to ease spying on the Trump campaign.