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.
After helping tens of thousands of companies set up voicemail systems, we’ve learned that there is no set of rules defining what makes a great business voicemail, but there are definitely some key points that you should aim to hit. Before you hit the record button for your voicemail message, take some of these tips into account:

While covering all of the relevant information, aim to keep your voicemail to about 20 seconds. You definitely should not ever record a business voicemail longer than 30 seconds, and anything less than 10 seconds will typically mean that you are either speaking too quickly or aren’t providing all of the required information.
“Hello, you’ve reached the confidential voicemail of Dr. Barb LoFrisco, CounselorBarb, a licensed mental health counselor, licensed marriage and family therapist and sex therapist. You may press # to bypass the remainder of this message. If you’d like to book an appointment, you can do so by using the “Schedule Appointment” button on the front page of my website, counselorbarb.com. If this is an emergency, please hang up and dial 211 for the crisis center or 911 for medical personnel. I am currently on the phone or in session, so please leave me a message. Your call is very important to me and will be returned within 24 business hours. If you are a current client please remember my 48 hour cancellation policy. Thank you for calling and I look forward to speaking with you.”