Where is the downstairs socket compared to the upstairs one? If it is directly below then you shoud be able to attach a draw wire to the upstairs cable and drag it back down through the wall and then attach another cable to the end of the draw wire and pull both cables back up again.
If you have a brick external wall there is normally a gap between the frame and the brickwork with enough room to get a cable through using a length of builders string and a sinker or another small weight on the end of it. If you can get hold of a length of plastic striping that they use on particle board flooring to join it together that stuff is excellent for getting into tight places. Just drill a small hole in the end of it to attach the cable and drag it through the wall.
If you are thinking of using the existing cable you only need 2 wires for the phone but you will more than likely end up with all sorts of interference using the same cable as the phone line.
Also if you are looking at using the adapters that plug into the power points be careful as from what i hear they have to be both on the same circuit to work.
hope it helps. Allen
Last edited by born2fly_au; 26th Apr 2007 at 21:59.