That is a 'Catch 22' in that when I ring 1571 it responds by saying I don't have the 1571 service and therefore I'm unable to take any options. Also, because the web site is stating it can't access my account I guess it's a case of having to ring on the 0800 number and try to set up 1571 that way.
I've just re-read your original post. Are we all missing the point here and your problem isn't actually with the answering service at all but rather that your phone doesn't ring?