"...it lets you know if there's likely to be any-one at the other end..."
If there isn't, I leave a voice message or send an e-mail. That way, my message has been delivered and I can move on to the next task.
I can see your point about a help desk, but support calls where I work are rarely of a nature that the caller can't wait for a few minutes. I realize everyone has different work environments, but maybe that's why these tools don't fit me.