Who I have the same questioned this solution

I am having the same problem.  I have called Customer Support and they told me that I am on Basic Visual Voicemail. I don't know if that is correct or not.  I can't use the voicemail app on my phone so I am suspect that I don't have basic visual voicemail.

Secondly, there is no option for me in the voice prompts to turn the notifications back on.

The only answer tech support gave me was to rebuild my voicemail box.  That would delete all of my saved messages, an option I'm not really keen on.  I have a feeling that it won't fix the problem either since my wife's phone started doing this also after the system upgrade.