OK, I will try to keep it simple for you. First of all, you should try to talk about her more. When talking to women, most guys will dominate the conversation talking only about themselves, and that is horrible. If she asks you a question, answer it, but then redirect the conversation to her. Ask about her. This shows that you are interested in her life and you care enough to talk about her...
A woman will often give you cues as to what she wants to talk about. For instance, if you make small talk about the weather and she mentions how much better it was back in Florida, you can then talk to her a little bit about that. The key to a good conversation is to LEAD (obviously most girls won't lead it for you), but let her do most of the talking. If you're doing well, she'll be doing a lot of talking. If there are a lot of awkward silences, you need more practice. It is your job to direct the convo and pick what to talk about, but let her do most of the talking if possible. Of course, during all this, you should tease her, make her laugh, and when the topic gets kinda played out, think of something else to say that can open up a new convo. It can be challenging if you know absolutely nothing about her, as is the case with me usually, haha, but a little bit of coc-ky funny and some talking about her will go a long way. After all, what is the point of the call if you don't want to get to know each other a little?
What you don't want to do is bore her, which is where most guys fail. This will lead to a fast death. She likes to talk about herself a lot, so you can use that a lot of the time. As I already say, tease her, playfully make fun of her, and above all else, make her laugh. If you talk to her like a friend, you will become her friend. She already has PLENTY of girlfriends to talk to all day on her cell phone, but you have to show her that you're a man, and talk like one. That said, don't come on too strong either, which will lead to an equally fast death. You simply give her an opportunity to meet up with you somewhere, nothing more. You don't have to make a "date" on the first call, but you need to meet her asap because telephone tag can only last so long.
Let's see, what else..... oh yeah, talk slowly but not like you're high or something, just calmly and in a relaxed manner. Slow yourself down a little bit and it should help some with the nervousness. Don't talk too softly. Talk with authority. You made the call, you're confident, and you are completely at ease with the situation. Hope I helped you some.