Many businesses try to save money by selecting an amateur voice actor from inside the company to provide a recording. The trouble with this is that: The “actor” is typically an amateur with no voiceover experience. The company doesn’t have personnel with directing voiceover. The “actor’s” goal is to finish quickly and get back to work, not make the best-quality recording.
This script comes from Donato Diorio and it's quite simple and to the point. But it still does a great job of highlighting prospect benefits and the follow-up plan.

People have short attention spans these days, and you should always craft your communications for the lowest common denominator with something as universal as your voicemail.
To provide as much helpful insight to businesses as possible, we've compiled the top must-have information that should be included in your office's temporary voicemail greeting. 1. Your voicemail should be timely and relevant to the situation at hand. 2. Indicate if you’re working remotely or changing office hours temporarily. 3. Set clear, reasonable expectations for your callers about when they should expect to hear back from someone. 4. State clearly what types of matters are urgent and give examples of issues that are not. This way, patients and callers can have a good understanding of what to expect about call-backs and reduce unnecessary follow-up calls. 5. If you have made disruptions to existing appointments, you should note this, as well as how you plan to reach people to reschedule.