My first exposure to the calls was clips from some of Tom's sound collages. I learned where the source of those calls at this board. (I think someone posted the whole thing here as MP3s).
Anyway, thanks for posting that link- I remember searching for back story on the calls several months ago and nothing turned up. Nice to have a little more perspective on it... but at the same time I share the expressed sentiment of wanting the real Derek to remain a sort of a mystery.