Voicemail set up

I have had my phone for 4 years (iphone 7) and the voicemail didn't work when I first got it. I called AT&T and they were able to do it over the phone. It worked for a while until I did an update and then it went away. I have tried the keypad and held down #1 and then it says that I need to enter a password. I don't remember ever setting up a password. What else do I need to try?