I was curious about the state of some of the technology in the 20s, so I figured I’d lump anything useful that I found into a wiki page.
The 1920s were a period of continuous advancement in telephone technology, beginning with the first completely automatic switching office, established in Omaha, Nebraska, in 1921. During that same year the first deep-sea telephone cable was laid, between Key West, Florida, and Havana, Cuba. (It is not to be confused with submarine cables for telegraph signals which had been laid since the 1850s on the ocean floor.) In 1926 American telephone transmitters and receivers were first placed in the same unit, the handset, while in 1929 telephone linemen began using the power-driven auger to bore holes for telephone poles—a great advance over hand digging.