I'm not sure about the quality issue? It's possible the bell wire might
make a difference on an ADSL setup, but I'd say probably not in a normal
domestic setup.

The problem is that when you run extensions with quality twisted pair cable,
the signal (2+5) is pretty good, with good interference rejection etc. When
you add the bell line, this signal is unipolar and very subject to crosstalk
and interference, which then gets injected back into the signal, causing
problems.

Obviously, with small extensions and a quiet electro-magnetic environment,
this is often not detectable, but it can become quite troublesome if neither
applies.

Christian.