Try what I did in my exams this year - talk down their importance

It works to reduce nerves. When you get anxious the messages going across your brain don't find their way round as easily, or something like that

That's why keeping yourself calm is important for maximising performance.
Keep reminding yourself, if it doesn't work out, maybe it wasn't meant to be, you can learn from the experience, and something better might come along. As someone with social phobia, you'll be unlikely to do anything to make her hate you, so you'll at least probably keep her as a friend. This should make it easier to be natural.
On the phone, if you keep things in perspective, what needs to be said will come to you. If you keep wondering what would impress her most, she'll get a false picture of you in her mind so that whether a relationship forms or not, it'll never be the real you she liked/rejected. Imagine yourself in three months time. However you behave now, you'll probably feel a need to keep it up then. If you've only ever said and done what you felt like (ie, been yourself) then that's all you'll have to do in the future. To keep up an act with people in the long term is worse than to know you were yourself and it didn't work. At least then, you'll know you just weren't suited and so won't wonder what might have been.
Hope that helps
