Even after resetting my iPhone 12 is not receiving or making calls

I have the same issue as Chamalee$ explained in Feb 2024 and IdrisSeabright helped with ... I cannot get a call or call anyone. Having gone into Apple community I saw I need to turn off the call forwarding but I get the spinning wheel if I try to reach it - here is their conversation

Even after resetting my iPhone is not rec… - Apple Community

Could anyone help me... I am stuck having spent quite a long time searching for the answer. I am using O2 as a carrier and they do not seem to have an answer on their website... and it is very hard to speak to them (except if you want to buy something). Many thank. JP