If you are asking, yes. It is called VoLTE , but I think both services are linked and if one isn't working most likely the other isn't working either.
I bet it is... O2 need to hurry up and solve the issue.
I am affected by this also. Non-O2 supplied S20+.
The workaround to boot the phone then put the sim in has worked for now. Fingers crossed there is proper fix soon.
Glad that wifi calling stayed on for you. For me (S20, not O2 firmware but with an O2 pay monthly sim) it disappeared again after the Jan update.
I still have to power off, remove sim, power on then reinsert sim after each restart.
In my case I don't need O2 firmware to enable wifi calling and VOLTE and it worked fine for six months until the December update. I had hoped that the January update would fix it but sadly not.