I used to never use T-Mobile apps, just do not trust them anymore after 14 years of service. BUT, I used an app VM for years and suddenly when they got big, they began charging enormous rates. I decided to give Visual Voice mail by T-Mobile a try. It's free and had much better features than the app I was using, like the poster said above they show you a photo of who has left the VM, sometimes the words aren't exactly right, but then you can just hit play and listen. It's so much better than adding more apps to your memory.
Hope this helps.