Just reading the above, and if I understood the point mentioned correctly, my information from my past, BT lines, unless they are loaded junction plant, which I doubt now exist, had an impedance of 1K2. Unloaded cable is just one great capacitor, which makes it a terrible transmission line. Unless we go back to the stone age and use old fashioned cadmium copper wires separated a few inches apart, having inductance and capacitance etc in a matched situation then the line would provide a 600ohm impedance. So using 100 ohm as a test line may not be totally accurate