There are lots of longer cables for sale, but really only any good between your PC and router, from the router to your wall socket as I said before should be no more than 10m, not saying it won't work, but if you get probs and you call your ISP or get an engineer out and they find out it's over 10m then they are well within their rights to say "tough luck"
Basically the desgn standards for xDSL ctate the router/modem/TA should be no more than 10m from the xDSL termination point....i.e. your wall socket
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