My voicemail has numerous issues

I have an iphone XR.  I came to ATT at the beginning of September.  I set up my voicemail and put in a voicemail code - but now it is saying my voicemail box is locked.  I tried to reset the password online, but couldn't find where to do that (I have prepaid if that matters).  I also can't set up a personalized greeting without getting an error (it lets me record it but not save it).  The voicemails also don't show up in the visual voicemail either.