I would be interested in hearing how people would field this question, at interview, "If we could not offer you the paid position, would you consider doing it on a voluntary basis?. Your answer will have no influence on our decision, of course!" This happened to me when I went for an intervention role. This sort of question seems a bit Catch-22; you're damned whichever way you answer. I answered the question 'yes', and I got offered the job but not the pay!