I'm having some issues getting visual voicemail to work using GrapheneOS on a Pixel 7. This is a brand new line that I got from US-based carrier Tello Mobile, and they do explicitly state that visual voicemail is a feature they support. I have previously used another line under Tello on iOS and there it does work without issues as well.
I'm using the Google Phone app for this, which appears to also be the recommended approach. I do not have Google Play Services installed, but from what I've gathered this should not be a requirement. I've tried contacting Tello, who did a reset on their end, but still no luck - the app simply gives me a generic message about visual voicemail having failed to set up correctly. Regular voicemail does work, but the visual voicemail feature does not.
@matchboxbananasynergy kindly linked me the GrapheneOS FAQ page that talks about carrier compatibility, though looking through this information I haven't been able to make any progress here. Tello uses T-Mobile's network, which is not mentioned as having these kind of compatibility issues.
I'm suspecting maybe this is simply an issue of compatibility and it can't be made to work under this carrier, but nonetheless I wanted to ask if anyone has thoughts on anything else I can try. I'd prefer to use the phone app to also manage voice mail rather than a separate app for it, but am open to any suggestions.
Thanks!