iPhone no longer displaying new voicemail messages

My iPhone 5 has abruptly stopped notifying me of new voicemail messages yesterday. I did not update any software recently.

Not only am I no longer notified of new voicemail messages in visual voicemail, they are not listed. Does anyone have any ideas?

I only learned of the problem when a friend bragged about the humor in his message to me. I looked but could not find the message. I then left myself a voicemail on my iPhone from a landline, but have not been notified of the new message.

Please help!

Potentially pertinent details:

I have reset my network settings.

I have rebooted my phone.

I have deleted old messages.

I am on AT&T.

I am using iOS 7.0.