Several people I know have been having the same issue on IPhones 6S, 8 and 8+. I was told by T-Mobile that it is a IOS 12.1 software issue. Had the same issue last year with IOS 11 that was finally resolved with an update. Looks like they broke it again in IOS 12.1. I hope Apple pushes a update soon.
I had a chance to chat with Marissa to see what the ultimate fix was for the VVM (visual voicemail issues). From talking with her it sounds like this is a provisioning issue between the Voicemail to Text feature and the Visual Voicemail feature on iPhones. The solution was to remove the voicemail to text feature in our backend system to allow the visual voicemail to work. I recommend chatting with T-Force via Facebook or Twitter to have them review your account and features.