Firstly, you'll need a compatible handset. Most smartphones offer Visual Voicemail capability.
Visual Voicemail relies on a data service to work. That means a connection to Wifi or a mobile data signal.
It's therefore important, when not linked to WiFi and to ensure continued availability, that your phone's data settings are correct.
Visit https://www.plan.com/apn-settings to correctly configure your mobile data settings to our data service.
Visual Voicemail Set Up For iOS Users: https://support.apple.com/en-us/HT201436
Visual Voicemail Set Up For Android Users: https://mobile.spectrum.com/support/article/360001296667/samsung-visual-voicemail