Don’t ask her for it.

Yes, this doesn’t sound right, does it ?

But the best way to get a number (a REAL number) is to not ask but rather structure an opportunity for her to get with you, after you’ve already done the work of fascinating, attracting, and seducing her. Once she is excited about you, she will fall over herself to give you her number once you’ve structured an opportunity for her.

You can do that with some NLP negation (you: “Hm, I’ve got to go. It’s been nice talking with you. It’s too bad we won’t be able to have an interesting conversation like this again…” her: “Of course we can, here’s my number!”) or you can do it by asking her a to provide a suggestion as to how you could hook up again (you: “It’s been nice talking with you. Can you think of a way we can get together and do this another time?” her: “Sure, here’s my number!”).

Source : Newsgroup at alt.seduction.fast
ASF Faq

Additional reading : How to Get Woman’s Phone Number