If a phone rings once and then goes to voicemail or rings only briefly, it You can't definitively know the cause of a phone barely ringing and going to voicemail because neither a phone company nor the phone's operating system provider will tell you if your phone number is blocked.
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.

Updated in iOS: New features of Voicemail is how we can record or play it for later. Keep your iPhone updated to the latest version. and get Voicemail transcript features. Activation Step to Set up Voicemail on iPhone Form your iPhone home screen navigate Phone app then Tap on Voicemail. Tap Set Up Now. Now Enter the Password then Tap Done Remember – Enter password must be between 4 to 6 digits in length. Re-Enter the password again then Tap Done. Tap Custom to record a greeting. Would you want to use default Verizon greetings then tap (check) Defaults? Tap Record to begin.
Now select “Greeting” in the upper left of the screen. Tap “Custom” and then “Record” to record your personalized voicemail greeting.