Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.
Creating a professional voicemail greeting isn’t complicated, but you need to keep a few things in mind to ensure success. The following tips will help: Be friendly and welcoming - let your company's personality shine!; Have a clear voice, speak at a slow to moderate pace, minimize background noise; Ensure the greeting is human and approachable; Keep the greeting short and informative; Ensure the greeting doesn’t sound robotic or unnatural; Show your gratitude for the call by saying thank you; Manage expectations by clearly stating when the client can expect a callback.

The Best Way to Set up a Voicemail Greeting. Normally, small business owners use a landline or mobile phone number to manage calls. While it may get the job done, they do not allow you to properly manage business calls or set up an effective automated voicemail greeting. The best way to manage business calls is by using a business phone service.
For instructions on setting-up and listening to voicemail on Skype for Business select the following options: Set-up Voice Mail Listen to Voice Mail in Skype for Business Listen to Voice Mail in Outlook SFB Voice Access Commands Using Your Skype for Business Phone Click the Keypad icon. Click the Voice Mail options icon. Select Set-up Voice Mail from the menu that appears.