In reply to a post by Zarjaz:

This is incorrect information.

Plenty of examples of bell wires (and more) being terminated on wires from the feed cable. Nearly always the work of electricians who have installed wiring whilst the property is being built. In their defence, they are most likely working on lines without a live service on them ...

Not only that, but (as you know but didn't point out), for an extension to have three wires then the master must also (normally, and pre-geekfix) have three from its faceplate.

Most premises with properly installed extensions will have three at least, to T2, T3 and T5.