The old Long Lines network remained with AT&T , but its internal routing became non-hierarchical with the introduction of advanced computer - controlled switching .
The PSTN in the United States was essentially restructured with the 1984 divestiture of AT&T .
The old Long Lines network remained with AT&T , but AT&T 's internal routing became non-hierarchical with the introduction of advanced computer - controlled switching .
The PSTN in the United States was essentially restructured with the 1984 divestiture of AT&T .