This one really bothers me! I just spent a couple hours with Verizon support to find the truth. After battling tier-1 support for 90 mins I actually got through to the tier-2 support team (yay!) .... I spoke with a lovely well spoken woman who knew what she was doing and she put me on hold for about 5 mins while she looked into the matter. The answer ... The Verizon Voice Mail servers no longer have the default option of notifying customers when their mailboxes are approaching full (90%). They just left this out of their new Voice Mail system. Sweet!
After lambasting her with all the obvious reasons why this was useful to customers I asked her why Verizon would do such a thing that flies in the face of LOGIC! She quietly and meekly said ... "We don't always get the why?" I told her this was breach of contract and that I was switching carriers ASAP .... She was very professional and honest so I thanked her for giving me exactly what I was looking for. The Truth!
. I have a 6s Plus. Yes, I've turned it off and back on numerous times since I found out I'm getting no warnings and my software is up to date. I have no idea how long this has been happening since the absence of a notification is the problem. I was made aware of it being full when my parents couldn't leave a message about 2 months sgo, but I believe it's been much longer since I've gotton a notification. 
