that line subscribers should be able to set an upper limit on their billing per quarter. It would be pretty straightforward from BT's point of view, and easy enough to provide subscribers with a PIN which they could use to override the limit if they so wished.
Let's remember that BT is a privately owned company, not some kind of government department which can be told what to do every five minutes. BT directors have a legal obligation to their shareholders to maximise profits, and it isn't the company's fault that criminals are exploiting the telephone network. Having said that, a responsible telecoms provider should be interested in stopping such activities, and the self-imposed upper limit is an excellent (and easy) way to do it for domestic subscribers.
That isn't to say that we don't all owe it to ourselves to take the necessary steps to stop dialers being set on our computers. It doesn't require a degree in rocket science, and we shouldn't expect someone else to nanny us through every conceivable danger.