I have had this phone 5 months and had nothing but issues with it.
I've ruled out SIM issues, apn problems, network issues. I've disabled WiFi calling, 4g calling and 5g and the issue is still there.
Phone calls are silent both ways with the cause being this device. I have factory reset it and still the exact same. O2 give basic advice saying the network may be busier than usual.
You can send it into O2 to be repaired but personally, I'd contact Samsung to do it.