Hi Voodoodood,
Visual Voicemail is a feature provided by the carrier, in this case - AT&T. The only thing we can suggest is to clear the app cache in the Phone app by going to Settings > Apps & Notifications > App Info > Show System > Phone > Storage & Cache > Clear Cache. You may have to complete the same steps with the AT&T Visual Voicemail app.
Note: This will not remove contacts or call history.