To specify the Auto Attendant activity periods or schedule, you will need to create Auto Attendant menus. They can have different greetings and different users/hunt groups/other objects assigned to phone keys.
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.

The basic rule of thumb is that callers should hear one of two things when they first connect with your voicemail — either an apology for not being able to answer the call or a “thank you” for having called. You can do both if you prefer, but keep it short and to the point.
When you hear the Dial Tone, call into your landline or office voicemail box. The rule of thumb is when you hear the Voicemails Forever dial tone, then follow your phone's instructions to listen to your voicemails remotely. This will vary from carrier to carrier and phone to phone. You may need to call your phone networks customer support for updated instructions.