Hello, you have reached the dental office of Dr. X. We are busy assisting a patient now, but will call you back as soon as possible. Please leave your name, reason for calling, and your number. If this is an urgent enquiry, please call 911. In this case, the dentist is giving a reason for not being available and they’re providing a way to prioritize the patients who need the most help. They’re also leaving an emergency number for the most severe of situations.
Promote your business to the callers while they're on hold. Inform them of your range of products and services, their benefits and how they can help them. Or just re-enforce your branding with some pleasant background music.

By presenting a clear, welcoming message containing relevant information about your business, you are showing callers that you value their time. If your recording is vague, confusing, or drags on, customers will lose interest and hang up the phone instead of leaving a voicemail message.
Whether you’re out for the day or are off for the week, the first rule of an out of office voicemail greeting is: let callers know when you’ll be back! You’ll save them the frustration of continuing to call and yourself the hassle of listening to redundant voicemails. It’s also a good idea to set the expectation of whether you are answering messages while you’re away, and who they can reach for assistance in your absence.