Sounds like you don't have 4G Calling (often called VoLTE on some phones) active. This means the phone will drop to 3G to make a call.
See this.
https://www.o2.co.uk/help/network-coverage-and-international/wifi-and-4g-calling#:~:text=4G%20Callin....
Sadly not available on PAYG.
At present there is no system for making calls over 5G
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
