Repeat your phone number twice. People seem to forget that the receiver of their message has to write the number as you say it. Don’t rush through it. Even when you say it slowly, it’s hard to get down the first time. So repeat it again, so they can check to be sure they got it down right.
With Visual Voicemail, you can see a list of your messages and choose which ones to listen to or delete. Learn how to set up and transfer your existing voicemail. Create a voicemail password, then tap Done. Enter your password again to confirm it, then tap Done. Select Custom or Default. If you choose Custom, you can record a new greeting. Go to the Phone app, then tap the Voicemail tab. Tap Set Up Now. Enter your password,* then tap done. Record a custom greeting or use the default greeting, then tap Save.

Simply speak into your phone or computer while recording a greeting with your business phone service. If you need to create your own audio file, the voice memo app on your phone should do fine.
Your own answering greeting should be short. Don't bother to say that you aren't available to take the call. That's a waste of time, and there's no point in restating the obvious. Instead, simply identify yourself and ask the caller to leave a message.