Hi @Rab-uk and welcome to our community!
Xperia 10 supports Wi-Fi calling and the option can be found if you open up the Phone application > tap the menu button (three vertical dots) > Settings > Calls > Wi-Fi Calling.
Keep in mind that the requirements below has to be fulfilled for you to be able to use this feature.
- your operator and subscription (incl. SIM card) must support Wi-Fi Calling for the device model you're using.
- depending on operator, your phone must sometimes be operator-branded and/or use operator-customised system software.
- your phone must be in range of a Wi-Fi network.
If you aren't able to find the setting in your device and since the device model supports it in general, get in touch with your operator / service provider and they will be able to provide additional information if they supports it for the specific device, if your subscription supports it and so forth.
I hope that helps! 
