@disherm, this feature can be country or operator dependant so it means that not all S9 / S9+ receive this feature. Some variants may receive it later and other may not receive it at all. What is your phone software version and country variant? To check it please go to Settings > Phone - information > Information about software and provide data visible in green marked areas.
Hi I have the galaxy A40 and a s5e galaxy tab which support the feature,although at first all I got was just messaging but it now fully makes and receives calls,I am in north east England on the three network. Hope this is of interest.