Choosing the proper time for a phone call means keeping the other person's feelings and wishes in mind. When you give a time when you're going to call, stick to that time--don't leave the other person waiting by the phone. Avoid calling during the work day, when he might be distracted by other things, and don't call late at night, when he may be in bed. Late afternoon and early evening hours are best, as are weekend calls.
When speaking with a potential romantic interest on the phone, it's important to stay positive. Don't dwell on troublesome things in your life, even if they weigh heavily on your mind. Instead, talk about things that interest you: the latest movie, good times with your family or other things designed to buoy the conversation. When the time comes to hang up, leave on an upbeat note, with a joke or a compliment.
Always have a specific purpose in mind when calling: either to set up a date or to speak about something in particular. It needn't be anything heavy (and probably shouldn't be), but a purpose helps you prepare for the conversation and chat more readily with the other person, rather than stumbling into awkward silences or inappropriate topics. Draw up a little list of topics and stick with them when you talk on the phone. Once the conversation flows more readily, you can probably play it by ear, but that core purpose helps you get over any early obstacles.
Sometimes when you call a person, she isn't at home, leaving you to face the dreaded answering machine. Relax, speak calmly and clearly, and put an optimistic tone in your voice when leaving a message. Stick to business; don't ramble or stumble over your words. Instead, state your purpose in a few short sentences, and make sure you leave your phone number as well as a good time to reach you. Once you've left the message, don't leave another one until she responds; multiple messages are unseemly.
While a special someone may be on your mind all the time, he does have a life distinct from you. He might not call back right away. Be patient and give him a few days; otherwise, you'll look desperate or needy. If he hasn't responded in four or five days, make one more call, and then move on; someone who can't be bothered to call you back isn't someone worth dating.