An interesting article from Bell System Technical Journal of June 1942 describing research into developing a transoceanic cable. Topics including describing the long-wave and short-wave radio-telephone systems then in use, the design parameters of a repeater, physical construction requirements, and a long discussion of bandwidth and making the best use of it in radio, land lines, and cables. Also discussed are experimental cable installations to test all of the above.
There's a description of a deviced called a "vocoder" which appears to be a crude form of voice digitization so as to pack more bandwidth within the cable. The problem was that inflections were lost. Much later on the Bell System developed a way to squeeze more conversations on a cable by filling up the natural spaces that occur within conversations.