Going old school with this question.
I, vaguely, remember a topic about JK cable.
Part of the discussion, if my memory is correct, mentioned that the familiar green, red, black and yellow cable was used for Princess phones. Red/green for audio/signaling/dialing while the black/yellow was the power pair connected to the external transformer.
Was JK intended to be used to wire only a single telephone line, or, was it produced to be able to support two lines from the get-go?
Due to the lack of twisting, crosstalk would be an issue for long runs.