iPhone voicemail does not display my messages, tells me to call voicemail

i have an iphone 6 running iOS 11.0.3. I recently received a sound from my phone indicating a voicemail, even though no phone call preceded it. When I check my voicemail, I see a blank screen with 'Call Voicemail' in the center. If I push on the message a phone pad comes up for me to call voicemail. I've never done this so I called my own number. It asked for my password. I don't recall ever setting up a password. I made some guesses but they didn't work. Why am I getting this call voicemail message and is there anything to do if I can't recall my password?

You should probably call your carrier and ask them to sort it out.

From this Apple Support article:

Only your wireless carrier can access and manage the voicemail account for your phone number. On your carrier's customer-service page, look for steps about how to reset your password online or over the phone.