Voicemail message says 100% full

I'm assuming you are using the Visual Voicemail on your iPhone 8 Plus.

My advice is to call your own phone number and log into your voice mail box directly on your carrier (usually by hitting the * or # key, then entering your PIN when prompted). Make sure there are no voice mails there. If there are, delete them. Then actually change your greeting in your voice mail by re-recording a new one. This can cause your carrier to re-sync things properly with your iPhone's visual voicemail.