He simply shortened a couple of the leads and put them back in. It worked.
The length was unlikely to have made any difference, but assuming the box used push-in IDC connections they may have been the problem. I was a BT engineer for 23 years, and regularly encountered poor/intermittent connections with this type of socket & terminal block. Almost invariably it was due to the wrong insertion tool being used. People would try anything, including screwdriver blades, as the "proper" tools were not easy to get hold of, and expensive for just the one job. Unfortunately this tended to permanently bend the two metal contacts apart, so they wouldn't bite through the insulation and make a good contact. These terminals are also not designed to accept more than two wires (or the third one won't make a reliable contact). Nevertheless finding 3 or sometimes 4 was not unusual!
The polarity shouldn't make a difference - the ringing current is AC, so phones have to cope in any case. IF there is a fault with the speech & dialling circuit it may be a problem, but that would be rare. As pointed out above, the filter is built in with a combined phone/ADSL socket, so can't be changed. Like Keef I've had faulty examples of these units. Can you try a plain phone socket on the wires, to see if that works O.K? A cheap, disposable insertion tool is normally supplied with DIY phone jacks/wiring kits, so there is no excuse to bugger up the slots now!
Also note that the bell/ringer circuit, which used to be supplied from the "master" socket is now part of each filter. This means only two wires are needed between master and any secondaries. Removing the existing wire (terminal 3) is recommended, as this will often improve the broadband speed.