Whether you're phoning her up or texting her, you should do three key things:
1) remind her of your fun personality that she got attracted to in the first place
2) make her chase you by using playful lines known as banters that help you get girls to flirt with you
3) when asking her out, always make sure that you don't sound desperate or needy. The key to getting girls on dates is showing them that you're interested in them while not being needy toward them.
By the way, I've written a lot of content about using banters to make her chase and also about how to create flirty text messages that can make her feel wildly attracted to you. If you want to learn more about these subjects, I suggest that you check out my blog pages about them at:
http://www.pickupflow.com/text-game.html
http://www.pickupflow.com/make-her-chase-you.html
Hope this helps you.
Bruno