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.

[Linked Image from elkonv.com]

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 ...

[Linked Image from elkonv.com]

Line 1, Green and Red
Line 2, Black and Yellow

not connected,

Line 3, Blue and White