I saw that the OnePlus 8 Pro is on the list of devices that support Wifi Calling and it does not appear to be working. I have tried rebooting the device and toggling to and from Airplane mode. Is there a APN missing or something else?
I had the same problem even with my iphone. There was nothing wrong with the phone. However, my wifi was creating problem. Once I changed the wifi, it worked. Please try to do connect different wifi and see if it works.