I cannot get voicemail on my new iphone xs. I get an alert, but if I open voicemail, it is a blank screen that says Call Voicemail instead of the usual list. When I try to call, it asks for a password. None worked, so I changed it in settings, but it still said the password is wrong. I have not used a password to access voicemails in at least at least a decade!
I solved it myself. I used the myATT app to change my password. That caused me to have to set up voicemail from scratch. Once that was complete, everything returned to normal. Even old voicemails are still there.