Hi. I tried using the alternative number (020 8371 6241) as well, just today (2008.04.14).
I got that "This number is 'temporarily' un-available". I didn't get the strange noises mentioned above, but I still didn't "get through".
I'm fairly sure that NHS must re-direct their 0845 (4647) number exceedingly regularly, due to the number of "alternative" numbers listed (none of which worked when I tried). I wouldn't put it past Her Majesty's Revenue Service to do something equally
dastardly, unfortunately.
(First post, btw.)