They analyze traffic patterns, and if needed, apply for a warrent to listen in.
Get the facts before your next hissy-fit.
They have NO BUSINESS having that data in the first place, and the phone companies had no business giving it to them. It is telling that Qwest refused and the feds did not resort to legal action to pressure them, as they did with Google to get search data.
But what if it isn't people with good intentions. Suppose you had been a friend of Ken Starr's? What could have happened to you.