After World War II there is a call for research and development to improve on the capability of the old No. 3 telephone set. In 1950, a new model, called No. 4, achieves design and performance levels that meet world standards. The high sensitivity of the new model leads to the nickname "the hi-fi phone," and contributes greatly to the reduction of cable diameters. Commercial testing begins the same year at six locations including the Marunouchi Telephone Office in Tokyo, and the final version of the No. 4 phone is ready by 1952. By this time Japan is abuzz with the Special Economic Demand of the Korean War and the entire manufacturing economy is in full gear. The streets of Tokyo witness a building boom, and the postwar restoration of telephone service is further accelerated by a new demand for telephones.

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