Originally Posted by Paul

Normally you "fish" it out with a hooked device (or just use a coathanger) via a small hole. How easy the run is will depend on what else is in the cavity.
Running ethernet is better than running telephone cable by a mile and the only thing close to it is the specialist ADSL cable.
Believe me, if you call most ADSL TS lines the first thing they will ask is "are you plugged into the main socket?"
Second thing they ask is "are you using any telephone extensions?"
answer yes to either and prepare to move things lol