That's exactly what I am about to do in the house we're signing for in few weeks (waiting for the attorney to give us an appointment).

When I saw the phone jack's in every room, all terminating down in the garage, I just figured it would be rude not too. Seeing as we will have 2Gb fibre, it makes sense

Using old phone lines is exactly what I did for my parents house, worked like a charm. Highly recommend if you don't need the phone lines anymore.