Is the FTTC extension cable just a 30m RJ11 cable or something else? The reason I'm asking is that the easiest way to get from the master socket in the hall to my workroom upstairs at the back of the house is the way the phone extension is run - back out to the front of the house, up the wall and into the front bedroom then through another hole in a wall to my workroom. Obviously this would mean chopping off one end then re-terminating - not something I'd ask the installer to do. Ideally, if this is possible, I'd try to get some external grade cable and make a complete custom job.