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.
Here’s a generic business voicemail greeting example: “Thank you for calling [company/person’s name]. We are currently unavailable, but if you leave your name, phone number, and message, we will get back to you as soon as possible.” The above greeting is a professional voicemail script used by many growing and established businesses.

A quick web search reveals a number of business options for transcribing voicemail message audio into a text format. Many service providers offer such transcription capability as part of the overall phone service plans they sell, allowing their business customers to have company-wide access to text-based voicemail without being forced to seek a third party for that functionality.
It’s more important that your callers feel like they can ask you for help on a subject they don’t know too well. A really easy way of simplifying your greetings is by using smaller words. Imagine yourself explaining what you want to a three-year-old. Your customers aren’t stupid, but they’re not going to always be familiar with typical business jargon. Make sure you’re adjusting your greetings to accommodate for the lack of familiarity, unnecessary details, and technical information.