Open the Phone app (the green icon at the bottom of your home screen), then go to the Voicemail tab. Go to Set Up Now. Create your voicemail password and tap Done when complete. Enter your password again and tap Done to confirm. To select a greeting, choose Default or create your own by selecting Custom, then tap Done when complete.
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.

While missed calls aren’t ideal, you can let your caller know you’re still there for them by having a great voicemail greeting. The best business voicemail greetings let your customers (and potential customers) know why you’re not available and how they can best get in contact with your business. They are also short and to the point. You usually want to keep your greeting between 6 and 24 seconds long so callers don’t hang up halfway through.
You can change your voicemail settings from the Skype for Business Settings page. You can get to the page by using one of the following methods: