I've tried a dozen combinations and none of them seem to work (even the most random I could think of).
Just spoke to the helpdesk, they told me to try again tomorrow morning after 9am as their online system may not be up to date, and the numbers I'm trying will probably be available tomorrow.
Here's hoping.
(side note: Fantastic customer support, answered in a couple of rings, plus I like the personal language used in the pre-recorded messages)