After exchanging phone numbers, it is customary to wait a couple of days before calling unless you've decided on a specific day and time. Prepare yourself before making the call so that you'll be less nervous and less likely to stumble over words or seem awkward. Preparation for the phone call includes remembering as much as you can about the person and his interests and coming up with questions to ask him. People typically like to talk about themselves, so the more questions you have, the better. Write down ideas if it makes you feel more comfortable. Also come up with a reason for the call, whether it is to ask the person out, to check up on them or to see if you have a connection over the phone.
Make sure to introduce yourself and remind her how she knows you. Wait for a reaction. If the person knows why you are calling, you will typically receive a positive reaction, and the conversation will begin. Start with pleasantries, asking how her week is going or what's new at her job. Make sure to be perceptive to the answers that she gives. For example, if she tells you that she ate Italian food for lunch and enjoyed it, you might ask her to tell you more about the restaurant. You'll also know that you could ask her on a date to an Italian restaurant.
Transition the conversation into something about the person's interests. Ask if she has recently seen a movie you think she'd like or if she's heard a certain song by her favorite musician. After a couple minutes of back and forth, if you feel that you've made a connection, it's time to get to the point of the call and ask her out. Once you've extended the invitation and she's accepted, discuss the details of the plan to meet, say something to make her laugh and tell her you will see her later; this should end the call on a good note.
If she declines your suggestion of a specific day or time because she is busy, tell her that you'd still really like to get together with her, and see if she suggests an alternate date or time for meeting up. This will show you that she is interested in going out with you. If she rejects your invitation without a reason and does not provide an alternate time or date, it's very likely that she's not interested in dating you. While you may be disappointed, you should end the conversation amicably.
If your call goes to voicemail, be sure to leave a message with your name and callback information and why you called him. If it is to ask him out, you can say what night and time you were thinking about. Give this person a reason to call you back. Do not leave the message ambiguously.