Heres my guide to flirting on the phone:
1. If you don't know what to say, don't say any more! I'm sure you have enough material to keep a good conversation going for at least 8 minutes. Keep her laughing and having fun, and as soon as you run out of things to say, don't let her know that. Tell her you have to go do something social. In other words, make it seem like you have something to do. By keeping her laughing and abruptly cutting off the conversation, she won't stop thinking about you. But if it gets boring, that's nothing but negative charisma for you.
2. If you're calling her, do so late at night, before you go to bed. Think about what you're going to talk about beforehand if you want to keep it going for half an hour or more. Come up with a few open-ended things to talk about. No yes/no questions.
3. Keep a cool-sounding voice. Talk slowly, this gives you more time to respond. Don't laugh too much, keep your voice deep, and don't start stuttering.
4. Practice makes perfect. Listen to other guy's conversations. Call some girls that you are comfortable talking to and just have conversations with them, and imitate those with the girl you like. It's important to make it seem as if though you aren't all that interested, and possibly calling because your bored. Confidence is THE MOST important thing in your success, and make sure you sound 100% comfortable talking to her on the phone.
Hope That helps!