For WiFi calls to work, your iD Mobile SIM or eSIM must be active and enabled in your device settings. Additionally, you’ll need to connect to a Wi-Fi network.
I’m not sure if you’ll incur charges. I’m also not sure if WiFi calling works when travelling abroad. However, theoretically, if you’re connected to WiFi while abroad and WiFi calling is enabled any calls to 01, 02, 03 and 07 as well as SMS to UK mobiles should be free since you’re not roaming on a foreign network. Obviously, any calls or texts to other numbers will incur a charge.
Perhaps try changing the network selection method from automatic in the phone settings to manual. Then, before travelling, select the iD Mobile network. This will prevent your phone/SIM/eSIM roaming on a foreign network but still allow access to WiFi calling.
Don’t forget to set your bill cap to zero (just in case).