Website: https://www.affiliatedunicom.com/nec-telephone-system-support/new-extension-setup-voicemail/
Android phones can access voicemail by holding down the 1 key on the dial pad until the phone dials your 10-digit phone number. You will automatically connect to your mailbox and be prompted to enter your temporary password; this password is the last 4 digits of your phone number followed by the # key. Start the Google Voice app. Tap the three horizontal lines at the top left of the screen (sometimes called a hamburger menu), and then tap “Settings.” In the Voicemail section, tap “Voicemail greeting.” Tap “Record a greeting.” How do I access my voicemail? To call your voicemail on an Android phone, simply open your phone’s dial pad and hold your finger down on the “1” key. You can also call your voicemail from a different phone by calling your own number and tapping the pound key. How do you check your voicemail?

Can’t record a custom voicemail greeting on iPhone? You need to try out a few troubleshooting workarounds to throw out the problem.
How to Record Voicemail Greeting on iPhone Tap the Phone app to open it. Tap the Voicemail tab. In the Voicemail screen, tap Greeting . In the Greeting screen, tap Custom. This is where you'll record your voicemail greeting and stop using the default... Tap Record and start speaking. When you're done recording the voicemail greeting, tap Stop . See More....