Definitely a one time pad - and therefore the answer to your question is "it depends on the randomness used". Sound sometime has surprising decryption properties in that starting and stopping long runs of data isn't always obscured properly by some schemes, so I'd also say "it depends on the exact scheme used for the frequency shifting."

This was at the dawn of the age of modern information theory, built on the shoulders of giants, but there are often some subtleties.

An example: if we imagine the encryption is perturbing a waveform +/- 30% of its frequency and amplitude at any one time, the encrypted bitstream is likely totally intelligible to the ear when replayed, even if reconstructing the exact bitstream is difficult.

Another example - if you make a block cypher that relies on prior blocks to seed randomness, then encrypting say 1,000 sequential blocks of zeros is very likely going to have some structure to it unless you've specifically designed it not to.

I'd love to hear some waveforms of what these calls sounded like encrypted -- it would be a fun project to work on decrypting them with modern tech.

Might be able to re-produce the signal in GNU Radio, I think there might be enough information in the Wikipedia article about how the modulation and encryption worked to get something similar going. The actual voice that came out was barely understandable lol: https://www.youtube.com/watch?v=OvR4qK0B--w&t=517s