AT&T initially had a monopoly, but when their patents expired in early 1890's, they quickly began to lose their dominance to competitors. Between 1890's and 1913, AT&T lost telephone market share by about 40%, from 95% to 55%. After the Kingsbury Commitment and, more importantly, the nationalization of the entire telecommunication industry during WW1, however, their competitors were effectively eliminated.

[1] UNNATURAL MONOPOLY: CRITICAL MOMENTS IN THE DEVELOPMENT OF THE BELL SYSTEM MONOPOLY https://object.cato.org/sites/cato.org/files/serials/files/c...