My IPhone 11 XR is not cooperating.

When trying to set up my voicemail, why does my iPhone 11XR show a Call Voicemail when I got to the Phone App & select the Voicemail icon? How do I set up my voicemail again? My carrier tried working with me to trouble shoot what the problem is, and nothing, they couldn’t fix it. Even the handbook says the same thing the Agent from my Carrier was saying & doing. Still nothing. Is there another solution? When I call my voicemail, it tells me “Hello, the person you have reached has a voicemail box that has not been set up yet, good bye.” Help...anyone?