Well at this stage O2 don't offer VoLTE (4g Calling) to PAYG customers. So on your version of Android you won't see the option until they do.
Whether O2 change that or leave their PAYG customers in the lurch we don't know.
At the moment when 3G closes, you will be able to use 4G data but calls will go 2G
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
