Practice your answers to anticipated questions with a tape recorder. This will help you see how you sound over the phone, and whether your voice shows interest and enthusiasm, or whether it is a dull monotone that will put the interviewer to sleep. Restate the question. Don't answer a question that you haven't fully understood. This will also give you time to prepare an answer and prevent long silences on the phone.
Dead space is much more noticeable on the phone than at an in-person interview.
Don't do this too often, though, as the interviewer will quickly become tired of repeating himself or herself. Answer questions courteously. A note of irritation or frustration in your voice is much more noticeable over the phone, and can't be offset by positive body language. Try to sound relaxed and confident.