Visual Voicemail isn't working

I got an AT&T PREPAID account about two months ago, but the Visual Voicemail doesn't work. My phone is an iPhone 7 Plus. It's an old phone I used with StraightTalk before I upgraded to an iPhone 8 Plus. I got the phone unlocked, and put my AT&T SIM card in it and got a new number. I set up my voicemail, but for some reason I can't see any voicemails when I switch to the Voicemail tab on the Phone app. I looked at the plan I bought ($65), and it said I had access to Visual Voicemail. But despite the many calls I've received from people who knew whoever had the number before me (plus numerous spam calls), I didn't see a single voicemail. It wasn't until I actually called the voice mailbox that I could access them. How do I fix this?