Here is a simple technique to increase the enthusiasm and positive image you project over the telephone: stand up. Whenever you are talking with a potential employer on the phone, stand up. It gets your blood flowing, improves your posture, and improves your response time.
Keep your mind focused on your phone call. Remove all distractions from your line of sight.
Its interesting to note that many telemarketing companies have come to realize that standing can actually improve their sales, so they often provide the telemarketers with hands-free headsets that allow them to stand and pace back and forth. It helps give an action perspective to an otherwise passive activity. So apply this same technique to improve your telephone presence.