It was agreed upon after the Cuban Missile Crisis in 1962, and was intended to help prevent an accidental war.

It was not a telephone, but a teletype machine at both ends. The unit in the USA was at the Pentagon, in the Soviet Union it was at the Kremlin.

The hotline was achieved with a 10,000 mile long cable between the two capitals, with encoding and decoding cyphers along the way to prevent the messages being read by unauthorized personnel.

The first message sent was sent from the USA:

The quick brown fox jumped over the lazy dog's back 1234567890.

The sequence uses all of the keys on the teletype machine.

In fact the line was rarely used. In 1999 a secure telephone connection was introduced between the two nuclear powers for the first time.