that independent telephony operators were permitted to "secure for their subscribers toll service over the lines of the companies in the Bell System." removing the barriers to a nationwide telephone system that would have no competitors. Local toll networks While the Bell System had a specialized division, Long Lines, to interconnect the local telephone networks of its Associated Companies, no such unifying driving force existed in the independent telephone industry. Telephone companies negotiated interconnection with neighboring businesses and built localized toll networks that addressed the regional needs of their customers. The interconnection agreements with the Bell System provided access beyond these network. Long-haul and transcontinental transmission Long-distance toll lines for transmission of telephone calls were almost entirely open-wire pair installations early in the 20th century. By 1911, the Long Lines network had reached from New York as far west as Denver, using loading coil circuits, but this distance was the limit for communication without amplification. The research efforts at Western Electric, committed to by Vail in c. 1909, into the principles of the electron tube recently invented by Lee de Forest, the Audion, and its efficient manufacture made it possible to build signal repeaters that extended the transmission distance of toll lines. In 1914, AT&T succeeded in the first transcontinental transmission line spanning between the Atlantic Ocean and the Pacific Ocean. This connected a large customer base in the far west beyond the Rocky Mountains to the AT&T Long Lines network. Open wire, while marginally increasing in installations in the 1920s, was increasingly supplemented with cable routes, experiencing dramatic growth, but also with carrier transmission, a new development which multiplexed multiple communication channels, at times 200 or 300 circuits, on the same physical cable medium. By 1925, the extent and quality of transmission lines in the nation was good enough, so that telephone subscribers could place telephone calls to almost anywhere in the continental United States. However, set-up times for calls were typically still long, and callers often had to hang up after ordering a call with an operator, who called them back when the circuit was established. In 1925, the average time to establish connections was still over seven minutes, but this improved to about two and one half minutes by 1929. The extensions of the nationwide interconnections led to rapid increase in traffic. In 1915, less than a quarter billion toll messages had been carried in the Bell