Telecom giant AT&T (NYSE: T) is best known today as a leading wireless provider in the United States, but it's also investing heavily in fiber internet. To expand it, a provider must start by laying down fiber in neighborhoods and near businesses -- a capital-intensive endeavor -- and then hope that enough customers sign up for the service to make its investments worthwhile. Right now, AT&T's fiber network reaches 19.7 million homes and 3 million business locations, but only a fraction of those potential customers have signed up.