An ordinary phone line is carried by a pair of wires from the exchange to the master socket. These wires are usually twisted to reduce interference (though very old phone lines may be untwisted).
Phone cable usually has multiple pairs (some older cables may only have one) so it can carry multiple phone lines, to provide spares for if a cable fail, to allow it to be used for wiring after the master socket and so-on. Typically in a domestic environment phone cables will be 2-pair or 3-pair. You may also see 4-pair cat5 used for phone wiring sometimes.
Most but not all modern phone cable uses a system where each pair has a primary colour and a seconary colour. see
http://en.wikipedia.org/wiki/25-pair_color_code . In BT spec cable* each wire in a pair will have rings of it's partner colour. So the blue/white pair will have one wire which is blue with white rings and one wire which is white with blue rings.
One exception to the colour code mentioned above is BT dropwire (used for pole to house connections), that has orange and white as the first pair and green and black as the second pair and there are no rings. It's possible that someone has extended an incoming dropwire with internal cable and matched the colours rather than maching the pair numbers. It's also possible that cajar's suggestion that the first pair was bad or in use for something else is correct.
The polarity of a pair doesn't matter too much, theres a correct polarity but the vast majority of equipment doesn't care if it's reversed.
Your not supposed to mess with wiring prior to the master socket at all but if you do then make sure you put things back as you found them.
* Other cable types may have other indications of the partner colour, for example cat5 typically uses a stripe on the white wires and no partner colour indiciation on the coloured wires.
edit: fix broken link
