Custom voicemail greetings are essential. If you don’t have one, it makes your business look unprofessional and a little robotic. Be sure to include something that people will want to hear when they’re calling your business. Inspirational voicemail greetings mean a lot ot your callers, it lets them know you care. When it comes to business, voicemail is the most important form of communication.
Microsoft continues to ride roughshod over people who have been users - at not inconsiderable expense - for YEARS. Skype itself is NOT free to use (even leaving aside my paid phone number) because it wastes hours and causes anxiety and frustration, each of which has a cost. Skype was never perfect but it had some degree of honesty before it was swallowed up by Microsoft.

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.
While a short recording may not seem like a place to win new business, the professionalism, courtesy, and clearness that you convey in your greeting will leave a favorable impression in a customer's mind.