Samsung Galaxy S7 Voicemail Headache

For the past year or so I've been having an issue where some days my voicemail on my Samsung Galaxy S7 (currently running Android version 8.0.0) will request a voicemail password, even though at no point have I ever set up a voicemail password on my phone. This has always been an annoyance for me, so today I decided to finally do something about it and try and remove the password from my voicemail permanently. All the guides online said I needed to reset my voicemail password to begin with (a password I never added and didn't know). I did so through the myAT&T website, and received a text message telling me my password had been reset to the last seven digits of my phone number. However, when I went to call my voicemail number with the new password, my voicemail now told me that the number I called had never set up a voicemail box, even though I had. So after trying and failing to access my voicemail that way, I decided to change the voicemail number to something random, and then back to the voicemail I had before, to maybe reset the system or something. But after I changed it back to the old voicemail number, when I tried to call my voicemail, I was told my call couldn't be completed as dialed. So now I can't even get to my voicemail box. I can't exaggerate how frustrating this whole process has been, especially as this is all stemming from a problem I had no hand in causing in the first place (I 100% never set up a voicemail password; my phone just started insisting some days that I had one, while other days I didn't have to enter one to access my voicemail). Please help me with this issue.