This system involved stepwise routing from one toll center to another one logically closer to the destination to set up each circuit.
[5] Altogether, the toll networks enabled operators to place calls directly to distant telephones in some three hundred cities.
For entering the destination codes and telephone numbers into newly designed machine-switching equipment, long-distance operators did not use a slow rotary dials, but a ten-button key set, operating at least twice as fast, which transmitted tone pulses (multi-frequency signaling) over regular voice channels to the remote switching centers.
With DDD, customers themselves dialed an area code followed by a seven-digit telephone number to initiate long-distance calls without operator assistance.
Activated first in 1951 for about ten thousand customers in Englewood, New Jersey, DDD was available in the major cities by the early 1960s, but was not fully implemented until the 1970s.