I agree with you that "who is this" can sound challenging with the wrong tone of voice (or right tone, if that's what you're going for). I generally say it in a friendly tone so that the caller doesn't infer I am being rude. I just hate using phrases that might simply be more "gracious" as opposed to more efficient wordage to what I am asking. As a side note, I answer my own personal cellphone, generally, with just "yeah?". Most people don't take too well to this; however, I tend to be a bit more gracious with a "Who is this?" at work.