Visual Voicemail

I just switched to ST I called before switching and was told I could get visual voice mail on my phone. Now that I switched I do not have visual voice mail, when trying to access my VM i get the message "call voicemail" . I have a Iphone 7+, I was told to call apple and apple says its a carrier issue. I have spent over 5 hrs on the phone with ST today trying to get this straight. I was told by ST to download the "voicemail app" i downloaded 4 different apps NONE of them worked!!! I am beyond mad, when I had ST before I had visual voice mail, what is the problem now. I am about to go back to AT&T this is insane!!