I have the same problem with a Pixel 8a. The carrier's website (rogers) says turning on wifi calling should take me to a page to confirm the address for 911 purposes but in grapheneos it just turns on and doesn't work. The rogers tech guy said 90% of bring your own android devices folks can't get wifi calling to work. Get a phone from rogers he said...
If you put your SIM card into your family member's regular pixel phone with google software, then activated wifi calling, then put it back into your grapheneos phone could that work?