Expanding on the simple approach above, let the caller know who they've reached. This will not only eliminate people leaving voicemails by mistake, but it will reassure the caller that they have called the number they intended.
It’s inevitable; a caller will go to voicemail eventually. When it happens, be sure to remind the caller what extension they are leaving the message at, and any pertinent information you’d like them to provide. In some cases, such as department voicemails, you might want an option for the caller to back-out and try a different extension.

A word of warning: These greetings will not do you any favors if you’re in the midst of a job hunt or work in a conservative industry. Always remember your target personas. If there’s a chance they won’t appreciate your sense of humor, opt for a straightforward greeting instead. “This is Bond. James Bond. Okay, it’s really [your last name]. [Your first name] [your last name]. I’ll get back to you as soon as I’m done helping M16 save the world — which will probably be tomorrow at the latest. Have a good day.” “Hmm. Gryffindor … No, Ravenclaw. Yes, you definitely belong in Ravenclaw. *Pause.* Okay, you haven’t reached the Sorting Hat — it’s the voicemail of [your name]. Please leave your name and number (and just for fun, the Harry Potter house you think you belong in) and I’ll return your call as soon as possible.” “Hello! You’ve gotten the voicemail of [your name]. Leave your name, contact info, and the answer to the eternal question ‘Which came first, the chicken or the egg?’ Anyone who gets it right will receive a call back.”
2. Mention Hours of Operation Upfront. Most callers expect their call to be answered. Once they realize that your office is currently closed, the most important piece of information they probably want to know is your standard business hours.