Re: I think "May I ask who's calling" is more polite. - Edit 1
Before modification by lyringlas at 22/09/2010 08:32:04 PM
"Who is this" sounds less gracious and like more of a challenge.
You could try, "And who may I say is calling?" That is a nicer way to ask the question that actually asks the question you want answered 
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 they don'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.
