A professional voice mail message should contain your name, so that the caller knows whom he has reached, as well as instructions for the caller to leave his name and number and an estimate on the amount of time it will take for you to return the missed call. You may also include instructions as to what the caller should do if the call is urgent.
Your professional voice mail message should sound courteous and upbeat. Make sure to speak clearly and monitor your tone when recording your voice mail message. Listen to your message after recording, and ask yourself if it is something you would like to hear if you were the caller. Follow a script, and read from it while recording so that you do not make any mistakes or forget any details.
If you have a professional voice mail service, your voice mail message may also include your current status, which will tell the caller why you were unable to take his call. This service can change your status in your voice mail message depending on what you are doing at the time of the call.
You can find some examples of professional voice mail messages at Grasshopper.com's blog on professional voice mail greetings.