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?
From another Ooma Office phone. Dial your internal extension number and wait for the call to roll to voicemail. Press [*] and enter your PIN. Select option 5 from the main voicemail menu. Select option 1 from the submenu to record a new greeting, or option 2 from the submenu to record a new name.

Twigby sells the phone, and the user manual may be found at twigby.net/manuals if you purchased it from them.
Website: https://support.8x8.com/equipment-devices/phones/general-phone-settings/how-to-create-or-change-outgoing-voicemail-message