Between 1935-1950, telephone companies made extensive use of shortwave long distance telephone. All of this was done using single sideband, just like on the wired circuits.
Various techniques were used to increase quality and capacity. Sometimes, one audio sideband was modulated further away from the center of the channel, to reduce cross talk with the other sideband. During World War II, a third audio channel was also used. This was accomplished by dividing the audio into two sub-bands, which were modulated inside the main sidebands and then re-constructed.
Shortwave frequencies were not as reliable as long wave, but they covered more distance with less power. Also, SW was less prone to static over summer months. Typical frequencies used were 9, 14, and 19 MHz.
Shortwave Long Distance Telephone Performance Improvements
Achieving shortwave long distance telephone performance faces two major challenges. The first relates to the ionosphere. Shortwave user experience propagation variation and selective fading. Engineers face difficulties in designing demodulators to cope with these effects. The second challenge is frequency stability and accuracy. To get good quality audio from single sideband, you need to control the final mixing oscillator within less than 10 Hz.
Both of these challenges were addressed using early forms of synchronous detection. Rather than using suppressed carrier sideband, the phone companies used reduced carrier. Typically, the carrier was reduced by 25-30 dB and served as a pilot tone. Receiver oscillators were locked onto this pilot tune, which provided automatic frequency correction. The result was good audio within a few Hz of the original.
A second approach was to actually use the pilot (reduced carrier) tone as the oscillator for demodulating single sideband. This required filtering, limiting and amplifying the pilot. Limiting was needed to achieve a solid BFO despite selective fading.
You can read more about these shortwave long distance telephone circuits here. All in all, much of the high performance single sideband technologies adopted by hams in the 1950’s was developed by the phone companies decades earlier.