You want to start out by telling your listeners specifically what you offer and how it will make their lives better. Keep this brief - you want to generate interest and excitement so that the listener will request more information.
Keep in mind that the main purpose of your voicemail message is to get the prospect to leave their contact information. Follow up and give them incentive by offering a free gift, like a 'how-to' report or an ebook containing information of value to them.
Keep your voice upbeat, like you are talking to a friend about something really interesting and exciting. This will keep your listener's interest.