Wifi calling works for sure on all pixel 6/7 which I can confirm through personal experience. Any difficulties that you have with it are likely on the NETWORK side. Does wifi calling work for you with the factory system image?
If not, then you need to check with your carrier to see if they support wifi calling FOR YOUR PHONE (don't tell them you installed grapheneos).
I can also tell you that I had to have all of my sim cards (freedom mobile) replaced with newer cards in order for the wifi calling to work, and on ONE card, I had to activate wifi calling on their website, which had something to do with service address for 911 emergency services.