Your prospects are busy people. They have many demands on their time. They don't always return calls when you want them to. Sometimes they "forget" to return calls. Don't get mad--and don't get even, either. Get focused. If they don't return your first call, wait an appropriate amount of time (anywhere from two hours to a day) and call again. If they don't return your second call, same drill. Same for the third and fourth calls.
Excerpted from Creative Selling: Boost your B2B sales