lah23 Visual Voicemail does not work on GOS for many people, supposedly, due using some old API for it. Will there be an overhaul of Visual Voicemail to make it more compatible with different carriers?
thmf lah23 some community member was very kind to write a guide just a few hours after your question: https://discuss.grapheneos.org/d/20931-resolved-visual-voicemail-not-working-in-dialer-app Please let us know if it worked for you.