Could a modem for DSL be designed that lives off dc power sent down the copper signal pairs?
There is 50V+ available already on the copper already. My father’s elder sister, Auntie Gladys powered a great big fat mechanical bell-clanger off it, and in fact her son probably still does.
If you designed some safety system at the home NTE end to make sure in-house wiring such as extensions were separated off, and little fingers were kept out, perhaps you could increase the voltage a bit too? Just so long as you did not electrocute BT staff working in the field of course.