I pulled up the Bell System Eng & Science history and there is an extensive chapter on radiotelephony and overseas calls. They go into good detail on the technical challenges of high powered transmitters, greatly varying originating voice signal strength, amplification, echo control, 2-wire to 4-wire conversion, antenna design and construction, signal propagation, etc. It is in the first volume of the series,
1875-1925. I strongly recommend it*. A large muncipal or college library likely would have it.As mentioned, they used both high and low frequency depending on conditions. Channel capacity was very limited. It appears that functions of radio transmission we take for granted today all had to be worked out by them. It amazes me that a tiny little cell phone can do everything (although at much lower power) they needed to do to send and receive a telephone radio signal.
They DID use a basic encryption to provide some privacy by inverting some frequency bands and "wobbling". As mentioned, a determined listener could bypass that but a cascual listener would not hear intelligable conversation. The book goes into detail on this process.
*I also recommend Vol 2 which covers 1925-1975. Lots of interesting stuff on switching development. Vol 3 covers military applications which I didn't find so interesting but there are chapters on military radio development. At the beginning of WW II they experimented with both AM and FM vehicle radios to see what was best. They also talk about the challenges of dealing with sub-contractors meeting demanding military specs. Equipment for the domestic market simply would quickly break in the rough military environment.