Definition of "hot line"

Later on, when he had loosened up considerably, he told an amusing story that he said took place during a Sino-Soviet border flare-up in 1969. "We had a hot line between the Soviet Union and ourselves then," he said, "but it had already become cold because the Kremlin never used it. At the time of the Chen Pao border incident, however, Kosygin picked it up and called us. When our operator answered, he said, 'This is Premier Kosygin. I would like to speak to Chairman Mao.' The operator, completely on his own, said, 'You are a revisionist, and therefore I will not connect you.' So Kosygin said, 'Well, if you will not try to reach the Chairman, will you please connect me with Prime Minister Chou.' But the operator gave the same unauthorized reply and broke the connection."

1978, Richard Nixon, RN: the Memoirs of Richard Nixon‎, Grosset & Dunlap, page 568