AT&T Inc. An American multinational conglomerate headquartered in Whitacre Tower in downtown Dallas, Texas. It is the largest telecommunication company in the world, the largest provider of mobile telephone services and the largest provider of fixed telephone services in the United States through AT&T Communications. As of June 14, 2018, it was the parent company of Mass Media Group WarnerMedia, the world's largest media and Entertainment Company in terms of revenue. As of 2018, AT&T is ranked # 9 in the Fortune 500 ranking of the largest United States corporations with total revenue