If you don't record a new greeting, callers hear the default greeting. To record a new greeting: Open the Google Voice app . At the top left, tap Menu Settings. In the Voicemail section, tap Voicemail greeting. Tap Record a greeting. Tap Record . Record your greeting and when you’re done, tap Stop . Choose what you want to do with the recording:
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.

It may take up to 5-10 minutes for the phone to reconnect to the network and register itself.
Checking voicemail on Android from another phone. If you need to access your voicemail because your phone is out of charge or you are unable to access your phone, for the time being, you may do so from another phone as well: Call you own number from another phone; Let the recorded greeting play before it asks you to record your message.