Connecting at wall doesn't work, but extending from wall does

[Lawrence Ward]

My study is currently on the opposite side of the house from where the telephone line enters the house at the wall. An extension line +20m long runs from the wall to the study. At the wall it is directly wired onto the incoming line, and in the study the line ends with a splitter, pots on one end router on other. Everything works.

I'm moving my study, and basically (and exactly) only removing the extension line from the equation. The splitter is directly connected to the line from the wall. The sync/ADSL light on my router just doesn't come on.

I've used a multi-meter to test the the pins on the jack I'm plugging into the router to ensure they are connected to the wire on the wall.

The extension is visible the entire length and I'm sure that the are no other devices spliced onto it and it is the same line coming from the wall to my study. Just to be absolutely sure tested the extension by putting the multi-meter at one end and shorting it at the other and listening for the beep.

I'm quite perplexed, I've got no idea why ADSL isn't connecting directly at the wall, where as far as can see it should be working. Does any body have insight into this situation?
Undoing all the wiring and rewiring the old setup, everything works again.