A great voicemail is one that invokes a call back or response. Voicemails that have high response rates are those that are delivered with a clear, concise, and compelling message. In general, most experts in the sales and recruiting realms can agree on these 6 basic constructs of a great voicemail.
Ninety-seven percent of calls received from an unknown caller will go to voicemail. Crafting a great voicemail will give you the opportunity to initiate contact with those 97% of people you would have otherwise never have reached. Take your time. As you try different messages, track which ones have greater response rates and use that information to further improve your message.