Ideally, a business should have a complete set of messages for every phase of their call flow from beginning to end. While voicemail greetings are an important part of the phone system, there need to be recordings for everything leading up to the voicemail as well (assuming there are other steps in the call flow).
A professional voicemail should be short and succinct. Simply introduce your company, ask the caller to leave their information, and let the caller know when to expect a callback. Once you record your greeting, you can easily upload it into the OpenPhone app. Simply go to your phone number’s settings, then choose to record a greeting, upload an mp3 file, or use text-to-speech.

Even in today's fast-paced world, customers don't like being greeted with an automated message. According to the New York Times, callers of a certain age are put off entirely if they are answered by a voicemail instead of a real person.
13. Hello, you’ve reached [business name]. All of our team members are busy at the moment, but if you leave a brief message, someone will return your call as soon as possible.