Since new phones can now ring without the presence of a ringwire, and householders desire for wires to be 'hidden' I'm surprised that single pair CW1308 (and cat5e) has not appeared in mass on the market. I would imagine it would be a thin cable which is easy to hide.
Just a quick reply to this...
I recon two-wire would be acceptable on the basis that PBX master sockets were used, phones using ring wires are still out there, and much used, and anyone who put in such an installation would create a fair bit of confusing on that point.
Also, BT is now using 4-wire again (seems to the old colour scheme as well) which is pretty tiny stuff.
Mind you, cable from different manufacturers alone, regardless of the no of pairs can make a big difference. I have a reel I bought which is decent 3-pair with a nice finish sheath which is the size of BT's 4-wire.
Then again, well manufactured 2-wire would be even smaller.
Standard cat5e being unshielded will still act as a ring wire antenna the same as CW1308. The reason being the "imbalance" caused by the utilisation of only one wire in the pair, even though the cat5 has more twists it will behave in a similar way.
As long as it is twisted pair and solid core (not that nasty alarm wire or mutli-core stuff) it should be fine.
The 4-wire colour code is like so:
Pair 1 - solid blue and solid orange wires
Pair 2- solid green and solid brown wires
Blue - same as blue/white - terminal 2 (Line B leg) on LJU
Orange - same as white/blue - terminal 5 (Line A leg) on LJU
Brown - same as Orange/white - terminal 3 (ring) on LJU
green - same as white/orange - terminal 4 (spare/earth) on LJU