iPhoneAlley details a technique to trick the iPhone into using any custom audio file as your voicemail greeting.
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?

However, you have to decide which type of voicemail greeting you want, so when someone calls you it creates a good impact on others. Let’s get started with how to change greeting on iPhone. (Voicemail Welcome message)
Tap the Phone app to launch it. Tap Voicemail in the lower-right corner of the screen.Find the voicemail you want to delete. Tap Delete (or, on some versions of the iOS, the trash can icon) to remove the voicemail.