I'm guessing if someone is asking for advice on having fear of holding a conversation over the phone, 9 out of 10 times he hasn't built enough rapport or held an actual conversation decent enough to leave a winning impression. If you had, talking on the phone would be peanuts. Hence, in this case, wait a few days as usual, and call her up. At the start, you would want to hold a decent, C&F conversation with more funny than ****y. You will never go wrong with humour. ****iness is, however, fire at the early stages. You don't want to go overboard, especially if you are not a master at calibrating it yet. Gradually, with good response from her, add a pinch of ****iness, until you balance it well with humour.
Since you haven't had the inner game to leave a golden impression, it doesn't matter if you screw up on the phone. The key is to screw up and learn from it. After a few more nervous and jittery phone conversations, you will eventually become comfortable. You'd be a master at talking over the phone.
Once you become a master at phone conversations, you work on talking face-to-face, and this is the key. After you become good at this, nothing would be a problem anymore.
You realise that this is a learning process and you simply have to keep field-testing and experimenting every step. Take every woman from now as an experiment, nothing more. You don't want to put too much emotions into it, you're still learning. Take your time. After all, this is just having fun on your part. You learn more from failures than you do from victories. Don't be afraid of failing, embrace it instead. Go through all the steps without skipping any, and all your questions about women will be answered along the way.