TAT-1 (Transatlantic No. 1) was the first submarine transatlantic telephone cable system. It was laid between Oban, Scotland and Clarenville, Newfoundland. Two cables were laid between 1955 and 1956. One cable for each direction. It was inaugurated on September 25, 1956. The cable was able to carry 35 simultaneous telephone calls. A 36th channel was used to carry up to 22 telegraph lines.
The first transatlantic telegraph cable had been laid in 1858 (see Cyrus West Field). It only operated for a month, but was replaced with a successful connection in 1866. A radio-based transatlantic telephone service was started in 1927. It charged £9 for three minutes. A telephone cable was discussed at that time. But it was not practical until a number of technological advances arrived in the 1940s.
The developments that made TAT-1 possible were coaxial cable, polyethylene insulation, very reliable vacuum tubes for the submerged repeaters and an improvement in carrier equipment. Transistors were not used, being a recent invention of unknown longevity.