Voicemail is not working on your iPhone and you don’t know what to do. It’s incredibly frustrating when voicemail won’t work, especially if you were expecting an important phone call from a friend or family member. In this article, I’ll show you what to do when your iPhone won’t play voicemails so you can fix the problem for good.
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.

If Airplane Mode on its own doesn’t work, turn off WiFi or 4G and then try it again. This can also work.
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.