Need advice on what to say..

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Basically, this is my situation: I had been flirting with a girl in my class for 2 months. I'm very, very shy, so I was planning on waiting until the next to last day to get her screen name and number. Unfortunately, due to weather she wasn't there for the last two days of class. HOWEVER, she did at one point give her phone number out loud to someone else in class, but it was pretty obvious it was directed at me (probably out of fear that I might not work up the confidence to ask her). So, although I will never see her in school again, I do have her phone number. Now, my question is this. What exactly should I say when I call her? I know she's pretty much expecting it (or hoping for it), so she probably won't think it's creepy or anything. I just don't know what to say since I don't know her all that well yet, and I don't want to make a fool of myself (although she seems to be a very understanding kind of person anyway). Also, do you think she will be disappointed if I just ask for her screen name (instead of going ahead and asking her out)?