no visual voicemail?

I currently have an iphone 6. For at least two years, the visual voicemail function was so delayed in reporting that I eventually had to call and delete messages by phone. At some point in october, my aunt said my voicemail box was full, but, I never got anything in my visual voicemail... Again, I had to go delete them through a phone call.

Now that Apple just issued 12.4.9, I'm wondering if visual voicemail issues would be resolved? Someone on Facebook said that I might need a new sim card. Is that a possibility? I'm on T-mobile

Thanks