Usually a soft reboot by turning the phone off and on helps as you've experienced and or other troubleshooting tips as mentioned by @Blessed4life uusually resolved issues like these however if not then as this is a network driven function you may need to speak with your networks support department to ask for Calling Over Wi-Fi to be provisioned again and or an Over The Air Configuration message sent to your phone.
I glad to see a soft reboot worked @Barrie1
I wish you all the best.
Daily Driver > Samsung Galaxy s²⁵ Ultra 512Gb ~ Titanium Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
