When I call someone on the phone I sometimes ask, "Have I caught you at a good time?" Any answer to this question but, "No", is a, "Yes." If a person says, "No," then you should ask when you could call back. The best way to do this is to say, "Would you prefer that I call back this afternoon, or would tomorrow morning be better for you?" In this way you have made an appointment to have a telephone conversation. Any other response but a "No" means "Yes." If the person you’ve called gives you a, "Yes" then they have agreed to hear what you have to say rather than think about what they can do to get you off of the phone so that they can get back to work. This doesn't mean that you now have all day. It means that you have about 30 seconds to state your case. Make the most of that time.
You have to have an answering machine. Even though it seems that people never call back, they sometimes actually do. If they can't leave a message for you, they may call the next person on their list. It happens. I personally know of cases where people who sent resumes in answer to ads didn't get an interview because they couldn't be reached.