First of all, make sure WiFi Calling is enabled on your phone and then connect to a free WiFi network. It’s a good idea to do this while still in flight mode to ensure your phone doesn’t auto-connect to an overseas network.
To make a call, you need to replace the ‘0’ at the start of the number you want to dial with a ‘+’ and the International Calling Code. This applies even if you’re calling someone within the destination you’re in. For example, to call an Australian number you would drop the first ‘0’ and replace it with +61. Every country has its own International Calling Code which you can find on countrycode.org.
Please note, any overseas short-codes, freephone or premium numbers will not work when you have WiFi Calling enabled.