I've been able to verify that Voicemail isn't working, but far as I know, it's been turned on since I got the phone in 2017. However, when I looked at my voicemail list just now, I saw that no mesages have been recorded since July of last year, so maybe something turned it off and I didn't know it.
I've tried checking to make sure it's on, but all the instructions I've found say to go to the phone app (the one in the iPhone dock at the bottom of the Home Page, right?), select Voicemail at the bottom of the screen, and go to "Set Up Voicemail." I can't see that option on my phone. I've also tried going to Settings and selecting Phone settings there, but I can't see anything there to check if it's turned on or not. I did look in "Change Voicemail Password," but that's blank, so I assume I still have the same password.