Yes, I'm having the same issue. I have a physical SIM on T-Mobile. WiFi calling worked fine on a Pixel 7 running GrapheneOS. I moved the SIM to a Pixel 8a with GrapheneOS, and WiFi calls go through, but there's no audio on the 8a.
I've deactivated and reactivated WiFi calling multiple times. I've rebooted multiple times. Calls go through with audio just fine, if I disable WiFi calling and always use the mobile network.
Let me know if there's anything I can do to help troubleshoot this.