The concept uses frequency shifting to overcome the narrow bandwidth of regular telephone systems, extending the usable range by approximately two octaves.
[1] The input signal is sent on one telephone line as-is, or in some cases upshifted to provide extra low-frequency response, and sent on a second line shifted down by 3 kHz, which is normally the upper bandpass limit in telephony.
This results in greatly improved audio, adding a full octave of range, and pushing the total bandpass to 6 kHz.
However, the law of diminishing returns takes over, because each successive octave is double the size of the last.
FM radio quality would require five telephone lines to be installed, pushing the bandpass up 25% to 15 kHz.