Same here. I got the final version of oreo in February and there is no option for wifi calling. It used to work with Android 7.1.2. Come on Nokia! You are supposed to be better than this
I actually found the option for it after searching for a while. It's under the phone app -> settings -> Calling accounts -> (Click on the SIM card at the top that says T-Mobile) -> Wi-Fi call. Mine was off after the upgrade to oreo. Still haven't verified if it actually works though.