Once you click greeting, you can change it from default to custom and record your custom greeting. Press play to listen to it before saving it.
After creating, the Voicemail Greeting, you can test it by calling to your number from another phone, and see if it is playing correctly or not. Open the Phone App. On the lower screen, tap on the Voicemail option. Select Custom, with this option, you can record the personal voicemail greeting on the iPhone. Tap Record, to start recording the voicemail greeting message on the iPhone. Greeting samples are given in this article below, Once you are done with recording, tap Play to listen. Lastly, if everything is alright, and custom voicemail recording is perfect, then tap Save.

When your business is closed for vacation or holidays, it’s imperative that you set appropriate expectations for a callback and to provide an alternative coverage option, if available. The following greeting accomplishes this succinctly.
6. After listening to each message, decide what you want to do with it. The most recent voicemail message that you've received should start playing. When it finishes, you will be given some choices about what you want to do with the message.