By default, the global busy prompt and global unavailable prompt are applied to all extensions. If an extension user wants to use her/his personal greetings, you can change the prompts for the extension.
You can set when you would like to send callers to your voicemail. For example, you may want to send all calls to voicemail when you know you’re going to be out of the office and not taking phone calls. 1

Congratulations! You've recorded a professional voicemail greeting that your callers love. If you want them to keep loving it, you'll need to update regularly. Adding seasonal information and varying your message keeps your greetings fresh, which is especially important for businesses whose clients and customers call in on a regular basis.
Verizon only: Unfortunately, if you have set up the voicemail successfully but still unable to change the greetings of Voicemail from the Voicemail tab, then dial *86 and select option 4 to make a change. Step #1: Launch the “Phone” app on your iPhone. Step #2: Tap “Voicemail”. Step #3: Select “Greeting” on the upper-left corner of the screen. Step #4: In case, you are also using eSIM then select the second network to change voicemail greetings. Step #5: Select “Custom”. From here you can record a greeting. Step #6: Tap “Record”.