How do I call long distance using WIFI using Iphone XR?

If your carrier supports Wifi Calling you can turn this on in the Settings under Cellular.

If they don't support Wifi calling you can always use FaceTime, Skype or WhatsApp to make calls to other mobiles in different countries

Facetime requires both parties to have a data connection. It doesn't matter whether the connection is cellular, or wifi, or in the case of a mac, hard-wired Ethernet.

There is nothing special about making a WiFi call once it's enabled. If you are connected to a WiFi network, normally the phone will decide whether to use the cellular network or WiFi if the cell signal is weak. If you Put the phone into Airplane mode and then turn WiFi on, it will force the call to go over WiFi IF the device can successfully connect to your carrier from the network you're connected to.

That's a carrier service. Contact your carrier and implement your plan.

Regards

Giulio