indeed it does, but there are some interesting bits of info in the comments , they apparently have a patent on this for instance, and BT are just one non exclusive signed up vendor.
http://www.theregister.co.uk/2008/02...band/comments/
and the average users wont really care how it works, as long as it does, perhaps it might even encurage BT to lower line costs if you take more than one line.