Around here anyway, there is a slightly heavier JKT with 3 conductors, and in an older house, I have come across an old 3 wire twisted cable, with brown insulation, with red/green/yellow silk strands in the wires.
I think the 3-wire cable and phone cords were probably the norm years ago. I have an old Western Electric 500 phone dated 1962 which is fitted with a cord containing only red, green, and yellow conductors. The yellow would have been used where a ground connection was required, such as for party line ringing.
The old GPO 700-series phones were usually fitted with a 4-wire cord (red, white, green, blue), although some turned out specifically for PBX use were fitted with with only 3-wire (red, white, green).
Quad station wire was almost always used in the past, with cores of blue, orange, green, and brown. (Blue and orange carried the line.)
Since the early 1980s all new cable is the now-standard striped pairs, for residential use most often 2-pair or 3-pair depending upon the installer.
I figure on using at least 3-pair for new installs. The cost difference is negligible, and it never hurts to have a spare pair.
