When you first set up your Android phone, it may ask you to record your greeting for the caller. You can also keep the default greeting stored on the device. But, in future, if you want to customize the Voicemail greeting or want to create a new one, then you may need to delete the old recordings from Android. It may not be necessary depending on the Android device model & version; it’s good to know the process.
7. Once your voicemail announces that setup is complete, you can hang up the call.

On the iPhone, go into voicemail and tap "Greeting". Record a few seconds of something and tap stop. Before you tap save, use iFuntastic or SFTP to navigate to /var/root/Library/Voicemail on your iPhone. You'll see a file called "Greeting. amr". This is the audio you just recorded. Delete that file and replace it with your custom message which should then be renamed to "Greeting.amr". Tap "Play" on the iPhone to verify your message works. If it plays, tap "Save" and your custom message will be uploaded to AT&T's servers.
Follow the instructions and press that number. You will be asked to delete your current Voicemail greeting. Click on the ‘Yes’ button that appears on the screen if you want to remove that. You can save your new recorded Voicemail greeting or can use the default recorded option. From the home screen, select the Phone app. Note: Alternately, you can set up voicemail by selecting the Phone app then selecting and holding the 1 key. Select Keypad, then select the Visual Voicemail icon. Select Start. Select Ok. From any Home screen, tap the Menu key. Tap Settings. Tap Call settings. Tap Voicemail. Tap Voicemail again. View the voicemail number. Press and hold the 1 on your device’s dial pad to call and connect to your voicemail. Press the star * key to access the main menu. Press 4 to access the mailbox options. Press 7 to change the language of the voicemail prompts to either Spanish or English.