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.
Your voicemail says it all when it comes to missed calls. Therefore, a professional greeting is key when people leave you a message. People will form an opinion about your style of work just by listening to the quality of your voicemail greeting. And remember, always listen to your voicemail greeting when you finish recording to make sure it's acceptable.

You can enable or disable a greeting using the radio buttons at the top of the pane. The standard greeting cannot be disabled, however, the alternate greeting can be enabled in its place. Choose your own recording or use the system default.
d. Utilizing Ambiguity Over Clarity: When setting up your voicemail greeting be direct and to the point. Tell callers your information, a brief greeting, and direction—i.e. “Hi, this is Jim Shamalam (from Iron Industries). Sorry I can’t take your call right now. Please leave your name number, and a brief message and I’ll get back to you as soon as possible. Thank you.” This is ideal, as you inform callers and let them know what they should do to ensure a return call. A lack of direction can lead to callers leaving incomplete messages (lacking contact or other information) or even callers hanging up without leaving a message altogether.