First and foremost, it nearly goes without saying that it’s important to get a dedicated number specifically for work-related calls, but let’s just say it. Many of us working from home have had to deal with using our home phone number or getting an additional land-line installed. Those days of dial-up are long gone for most of us and it seems nearly everyone on earth now has a cell phone. There’s really no excuse not to have a dedicated work number these days. If you don’t have one, secure one today. It’s even easier than you think if you also consider options like Google Voice and My1Voice. You’d think this also goes without saying, but don’t leave the standard voicemail provided by your cell phone carrier or answering machine. It’s annoying. Everyone hates them. And don’t think that saying your name at the beep to customize your message helps. It’s just as annoying to hear the robotic voice that pauses as your voice interrupts the message to insert your name. Actually say your name You think I’m joking? I called someone just last week and when I got the voicemail it just repeated the number. I dialed by choosing the contact from my contacts and I’m not good with numbers. I wasn’t sure if the number was the number I mean to call, so I just hung up. It happens. Reveal specific information If it’s a typical work day and you’re just unable to answer your phone, it’s fine to say you’re currently unavailable. But if you’re sunning yourself on the beaches of Tahiti and have no intention of even checking your voicemail until you return to your desk in two weeks, it would be nice to let someone know. If they’re calling you they must need something. Let them know when you’ll be available. Give an emergency option Depending on what you do for a living, or just how accessible you want to appear to a potential employer, leaving an additional option for the caller to reach you often makes a good impression. Doctors offices do that all the time, right. If you can’t reach me this way, you can reach me THIS way. Or, you can reach someone else who can help you right now. KISS Keep It Simple, Stupid. You don’t want to speak like an auctioneer, but you do want to get as much information across as quickly as possible and get on to the beep. Your caller doesn’t need to know your life history, just the current facts and then sufficient space to leave you a message. Write it down Put some thought into exactly what you want to say. Don’t just hit record and play around until you’re satisfied that your message doesn’t suck. Think about it. Write down what you want to say and give it a try. Rearrange things, revise things. And, when you’re ready, hit the record button and give it a whirl. Smile Did you know that it’s believed that some 50 different types of smiles exist, from triumphant ones to bitter ones. Interestingly enough, there was a study a couple years ago where scientists used the phrase “I do in the summertime” to study the effects of smiling on person’s voice. Read all about in Smile – And The World Can Hear You, Even If You Hide. Or listen to the NPR show, Hearing a Smile in Tone of Voice. So close your eyes and think of your happy place. Think of something funny or amusing. Then, right before you hit the record button and begin speaking, SMILE. Callers will hear it. Just make sure you’re not actively laughing like the Joker from Batman throughout your voicemail greeting. Cut the comedy Smiling is a good thing. Acting like a clown, not so much. Cutesy comedic messages really don’t have a place on your work voicemail greeting. Be considerate Above all, consider the caller. A considerate message lets them know they’re important and that you’ll be returning their call. Even if the sun and moon revolve around your every move, your voicemail greeting isn’t the place for egocentric SOMETHING Listen to your messages and return calls
Website: https://technology.ku.edu/sites/technology.ku.edu/files/docs/training/SkypeforBusinessMac_Voicemail.pdf

You can also upload an unavailable greeting using the OnSIP app. After logging in, click on the Voicemail Settings gear icon at the far right. Make sure that the Custom Greeting is toggled to "on." You will then see an option to upload a .wav file of your choice. This recording will serve as your unavailable message.
8. “Hello, this is [your name] at [company]. Thanks for calling. Please leave your name, number, and the reason you’d like to chat, and I’ll get to back to you ASAP.”