I am in the USA using T-Mobile. Yes, voice calls work. Depending on when the device was shipped, it may require a modem firmware update. After that its a simple install of bm818-tools package, enable VOLTE and then reboot.
Pixel 4a 5G with GrapheneOS. Though it's finally starting to hold less and less of a charge, so I may "upgrade" to a newer yesteryear Pixel model.
Pixel 7 for GrapheneOS