I have the same issue some one said to restart the phone without the sim card in which I did and the WiFi calling option was there again but i since restarted my phone and its gone again I'm on 02 I'm so confused 😕
Did you use the beta at all? couple of things I've learnt. A 3 SIM works in the main sim slot and immediately WiFi calling shows up again. if I put the O2 sim back in then it goes again. also if I just put the O2 sim in the second aim slot, the hybrid one where the SD card normally is, then it works again! It's almost like O2 is blocking WiFi calling on my primary IMEI number in SIM slot 1, maybe because when I was on the beta they saw it as insecure, so maybe it's a safety thing their end?