Do callers get a "voicemail full" message? If so then dial voicemail from the phone itself calling its phone # and logging in to dialed-voice-mail. There you may find many ancient voice messages - delete them manually and voicemail app may start working. Enabling short-prompts will probably help with the slow manual deletions.
I have a Samsung Galaxy S10 and the voicemail app does not work on it. It doesn't show any notifications. I've called Xfinity and Samsung, no success. Noone knows why it doesn't work.