SIGSALY, an early digital speech encryption system, was developed by Bell Telephone Laboratories in the US in 1941/1942, and built by Western Electric in New York in 1943. Developed in consultation with Alan Turing, the system used the highly-secure One-Time Pad (OTP) encryption. The system went into service in April 1943, just two months before the invasion of Italy, and was used until at least 1946. Notably, it was used frequently for confidential talks between Winston Churchill and Franklin Roosevelt.
SIGSALY terminals, of which a dozen were set up around the world, were massive, consisting of 40 racks of equipment. Each weighed over 50 tons, and used about 30 kW of power, necessitating an air-conditioned room to hold them. Far too big and cumbersome for general use, the system was only used for the highest level of voice communications.
The first SIGSALY terminal was installed in the Pentagon rather than the White House, which had an extension line, so that Winston Churchill could speak securely with Franklin Roosevelt at any time of the day or night. "The second was installed 60 meters below street level in the basement of Selfridges department store on Oxford Street, London, close to the US Embassy on Grosvenor Square. The first conference took place on 15 July 1943, and it was used by both General Dwight D. Eisenhower as the commander of SHAEF, and Churchill, before extensions were installed to the Embassy, 10 Downing Street and the Cabinet War Rooms. One was installed in a ship and followed General Douglas MacArthur during his South Pacific campaigns. In total during WW2, the system supported about 3,000 high-level telephone conferences." (Wikipedia article on SIGSALY, accessed 7-2019).
For a much more detailed account of all the aspects of SIGSALY see Cryptomuseum.com at this link.