You can check to see the voice-mail settings by: clicking on the phone :mobile_phone: icon -> click on the keypad tab -> click on the (3 dots) -> click on settings -> click on voice-mail -> service provider. Check your voice-mail settings there. Hope that helps.
I verified the setting is working I used to have it intergreated into the phone app a couple phones ago and now have to use the T-Mobile app. I have noticed if I turn off the setting for notifications on the phone app the visual voicemail app does not always work