Check if your device is supported, then follow the advice in this guide
Edited to add further advice https://www.o2.co.uk/help/network-coverage-and-international/wifi-and-4g-calling
*The Game Is On*
Customer service need to enable it on your account first, if not done so already.
Call them on any of these numbers Guide: Coronavirus Community Help and Support
Once that's done, you need to enable it in your MyO2 first then find the settings in your phone menu.
Call as I said above then activate in your MyO2 as per the image I also posted.