Wifi calling on my iPhone 8 Plus used to work, I am a post pod customer and wifi calling no longer works for me. I am in an area where there is no reception/service. The fact that wifi calling no longer works for me is very frustrating. Yes I have gone into settings and enabled it. Yes I have restarted my phone. This has been an issue for too long and I am very upset.
Has anything else changed in yout setup recently? New modem?
Only thought I have is the phone switches between wifi and 4G automatically (I think) so perhaps disable your 4G on the phone to see if the wifi kicks in.
Other thought (that makes two) is you need to ensure your firewall settings are ok. Perhaps disable the firewall as a test?