So, the plan was simple. I've got a punchdown block in the basement that the phone lines are connected to. I punched two pairs down into the bottom of the block and ran that over to the new port. Then, stripped the existing cable going to the T1 port, and punched it down to connect with those. The upshot being that I now have one wire coming into the punchdown, and two coming out. The process is now easy, unplug the router, move it downstairs, plug it in, done.
Well, it didn't work. It eventually transpired that I had miswired the new port. I had reversed pins 1 and 2 by mistake. So, loree helped me rewire it. Aaaand it still didn't work. So, I figured, maybe the new cable is bad. I took it from a collection of 'scrap' cable I've got. But, I pulled it down today, and checked it with a continuity tester, and it checks out.
So, I dunno what's wrong with it. Maybe the old jack isn't wired the way I think it is, tho I find that hard to believe. Maybe the new jack's labels don't mean what they say. (why would your jack take pins 1 2 4 and 5 on one side and 3 6 7 and 8 on the other?)
Will need to spend time tomorrow figuring this out.