Open your Phone app. Tap Voicemail. Select Greeting. By default, your voicemail will be setup with your carrier's generic greeting. To record a custom greeting, select Custom. Tap Record to begin recording your voicemail greeting. When you finish recording, tap Stop. To make sure you're happy with your new greeting, tap Play.
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?

Website: https://www.xfinity.com/mobile/support/article/how-to-access-voicemail-android-phone
Jan 21, 2008 · Where "(your phone number)" is your 11-digit phone number (including 1 and the area code), and "15" is the number of seconds (from 5 to 30 in 5 second increments) that you want your phone to ring before going to voicemail. (The default seems to be 10 seconds.) You can thank me later. 😉 (Hat tip to Bradley!)