It sounds like your visual voicemail is giving you an error message about being unavailable. I would try the troubleshooting steps in this article named:
iPhone: Troubleshooting voicemail
- Restart your iPhone.
- Check for a carrier settings update.
- Update your iPhone to the latest version of iOS.
- Ensure that your voice mailbox is set up properly. You can verify this by dialing your phone number from your iPhone.
- Make sure that you have a good cellular network connection. You can verify this by seeing whether you can make an outgoing call.
- Call your iPhone from another phone and attempt to leave yourself a voicemail.
- Reset your network settings by tapping Settings > General > Reset > Reset Network Settings.
- Contact your carrier and ensure your iPhone is properly configured with the voicemail feature on your plan.
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