If you don't record a new greeting, callers hear the default greeting. To record a new greeting: Open the Google Voice app . At the top left, tap Menu Settings. In the Voicemail section, tap Voicemail greeting. Tap Record a greeting. Tap Record . Record your greeting and when you’re done, tap Stop . Choose what you want to do with the recording:
However, some cellular service providers allow users to access and update their voicemail by calling their own numbers from working landlines or phones on other wireless networks, so users whose accounts include such an option and who have access to alternative service could keep the voicemail greetings updated even when their own mobile phones cannot connect to cellular or internet service.

When you set up Voicemail for iPhone 7, you can enter the Phone app, and then tap on "Voicemail" to access to Voicemail to change for the desired information.
The very first thing to be checked is, have you set up the voicemail earlier or directly recording the voicemail greeting? If you haven’t already set up the voicemail and unable to record voicemail greeting, then go to the top of the page and learn how to set up voicemail on the iPhone.