Voicemail not working

My voicemail is set up on my iPhone 11 and used to work but now it says the person you are trying to reach is not accepting calls at this time. Idk how to fix this, I went to the AT&T store and they can’t fix it, all of my software is up to date, and this is also happening on my son’s iPhone 11 (we are on a family account). How do I fix this?