Data Encryption in Vacuum Tube Times
Back in the day, voice "scramblers", as they were called at the time, were strictly analog-domain. Typically, the voice band would be divided into a number of sub-bands by filters, and each subband would be modified (shifted in frequency and/or inverted) by means of oscillators and balanced modulators, in a manner similar to how SSB radio transmission works.

As long as the other end of the circuit had a matching configuration, clear speech would come out. But the signal in the middle would be essentially unintelligible. You would hear the loudness pattern of words and syllables, but not be able to make out the vowels and consonants.

The systems were finicky to keep calibrated; components drifting by just a few percent (or less) could make the system useless. Fortunately, the first mass-produced transistors and the first forms of digital telephony and digital signal procesing (DSP) came along shortly thereafter, and these were much more amenable to true encryption.