In many cases, an update to your carrier’s voicemail app or settings can resolve the issue, but don’t forget to call your voicemail number to check if it is set up correctly. Once you’ve set up your voicemail, you’re free to switch off when you need to. There are other ways you can stay in contact, however.
When recording a personalized voicemail greeting on your office phone, your greeting should include only the company name and your extension number. When recording a personalized voicemail greeting on your office phone, your greeting should include only the company name and your extension number. State your name and spell it out if you’re asking callers to follow up with written correspondence. State your company name and department name. Let callers know that you cannot take their call right now. Invite them to leave a message. What should I record on my work voicemail?

Website: https://www.att.com/support/topic/local-long-distance/topic_landlinehomephonesup-topic_voicemail1
Tap “Phone” on your iPhone’s “Home” screen and tap “Voicemail” to access your voice-mail settings. Tap “Greeting” and select “Default” if you wish to use the default greeting for your voice mails. Click “Custom” if you want to create a personalized greeting, and tap “Record” to say your greeting.