Users often don’t invest enough time into their messages, resulting in incomplete, unprofessional, or otherwise under-whelming greetings. Sure, crafting a greeting doesn’t sound all that complicated; however, there are a number of pitfalls users can fall into—i.e. informality, terseness, sincerity, lack of direction, and more. While none of these sound too catastrophic, they are often interrelated. As such, they tend to worsen any problem. For example, humor can cause informality, worsen ambiguity, and weaken sincerity. That being said, users should strive to avoid ALL these pitfalls.
Open your voicemail greeting with a 'Thank you for your call. We're not available right now by please leave your message and we'll get back to you within [specify time]'.

In an ideal world, voicemail would be unnecessary. There would always be someone available to take customer calls and no one would complain about their calls not being returned and/or rejected. But of course, this perfect world isn’t possible. That’s why it’s important to give your callers options when you are unavailable. For example, aside from leaving a message, callers can also choose to listen to some relevant information about your business (such as a list of your services, your address, directions, hours of operation, and more).
It's easy to get carried away in a voicemail and include more detail than is necessary. We've all been in that situation where the automated or pre-recorded voice on the end of the line goes on and on with more information than you can take in.