My voicemail says that it's full and when I try to check the messages, all I hear is this beeping noise. It's been like this for a few months.
Have you tried accessing the voicemail by dialing your phone number from another phone and interrupting your greeting to get to the voicemail system?
When I do that, it tells me that my voicemail isn't set up
Did you ever switch from an Apple iPhone to an Android phone?
No. I switched from a Galaxy s5 to a Note 4 recently. The problem started
on the Galaxy s5, if that makes a difference.