Today, international calling is routine, with a half-million transatlantic calls from the United States every day. But it took years of effort by AT&T scientists and engineers to make it so. They began to study deep-sea, long-distance submarine cable as an alternative to the telegraph and short wave radio in the early 1930s.

Transoceanic telegraph cables have been around for almost 100 years, but the more delicate voice signals with their higher frequencies could not make it across the ocean without a periodic boost to the voice signal. Thus the challenge was to design amplifiers installable three miles below the ocean surface and capable of operating for many years without requiring a repair. Built around an electron tube especially developed for this purpose, the amplifiers or repeaters, encased in flexible multi-metal housings, were spaced at 4-mile intervals along the cable. The cable also had an outer sheath of armor wires to provide strength and protection against abrasion and an extra copper sheath to keep out marine worms, which were known to plague telegraph cables.

Laying each of the two cables that made up the systems (one for each direction of communication) took weeks and was carried out during two consecutive summers. The first day of commercial service, September 26, 1956, saw a 75 percent increase in transatlantic telephone traffic.