I do a similar thing to a HB after building some sort of rapport with a girl, but I don't think I could pull off yours without building any sort of rapport or comfort. This is how mine usually go:
Me: Hey you're cute... are you friendly? (the emphasis was on the "friendly" part (I asked it suspiciously), and not on the "cute" part - so that meant that I was qualifying her for more than her apparent cuteness)
Her: Sometimes... teehee
Me: Sometimes? I dunno if I can work with that... what's you're name?
Her: HB
Me: Well HB, I'm PUA, nice to meet you. So do you go to school here? Did you just graduate? (or some other question)
Her: Yeah but I didn't graduate
Me: Ok, well I've gotta get going so I can rejoin my friends - gimme your phone number
Her: I dunno...
Me: Awww what's the matter? You didn't pay your phone bill or something?
Her: hahahahaha noooo... I'm kinda seeing someone!
Me: "kinda" seeing someone? Does it matter?
Her: Well kinda... you can facebook me
Me: So you want me to be all stalkerish? I don't think so. So when's the wedding?
Her: hahahaha it's not like that!
Me: So there's no harm in talking to a charming guy like myself, now is there?
Her: No

Ok, you can have my number
This is usually less than 5 minutes.
An opener that is also similar to this was one that I heard in one of David D's interviews:
The PUA approaches a 3 set of all HB8+. Doesn't say anything to any of them but is looking at all of them like he's judging them. PUA then points at one of the girls and says "You - gimme your phone number. I wanna talk to you". The other two HBs start primping themselves and the target says "ah... uh... ok"
That's like a completely alpha way of "asking" for a phone number. But maybe you can add something to yours like telling her that you saw her checking you out or something along those lines.