Hello!
I recently encountered this problem.
It is reproduced for the Canadian operator Koodo. However, I think that other operators have it too.
The problem concerns VoWiFi. The problem point is "additional post-registration" when activating the service.
The procedure is as follows:
1) you insert the SIM card, the operator sends the settings, you accept them.
2) you go to the SIM settings and activate VoWiFi (if it is not active yet).
3) a window opens, apparently it is a web view and you fill in the necessary details. It seems that this concerns E911.
4) Everything becomes fine.
The problem I am talking about is in point 3. The settings window does't open.
I tried everything. I used ActivityLauncher (try to launch VoWiFI configurations);
I tried Shizuku, because I noticed in the service menu - information about the IMS that calls via Wi-Fi are not available. But all these efforts were in vain.
Workaround
I put the SIM card in another device, registered VoWiFI, then returned it back to the Pixel and calls via WiFi started working.
With an operator that does not require post-registration for VoWiFi, there are no problems, everything works right away.
I think this information might be useful for GrapheneOs developers and our community.
thx.