Chopped7821 The carrier uses AT&T, in case anybody else has a similar issue.
I think where we are at present is that voicemail indications definitely work for many GrapheneOS users using the built-in dialer, including at least two Verizon users, and don't work for one user of an unknown AT&T MVNO using the built-in dialer but do work with the Google dialer.
If the name of the AT&T MVNO were available, perhaps some people might report everything is working fine for them using that MVNO... or perhaps some people might report that they, too, tried the same MVNO and eventually gave up.