Paul,
I thought I would revive this thread with some pictures of the older type South African telephone plugs. The RSA now uses the USA standard.
Each side of the outer two blades is electrically isolated plus two more surfaces in the centre, i.e. a potential total of three lines.
I know you'd want to peek inside so ...
Line 1, Green and Red
Line 2, Black and Yellow
not connected,
Line 3, Blue and White