No problem. Always happy to help. I took a few screenshot on my iOS 17.5.
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Salt is my home line and Kingdom is for travel. As you can see, both lines are turned on when I am on travel.
Cellular Data is currently off - this is where you make the switch to the travel line once you are abroad.
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If you select the travel line in cellular data, you will not use any of your home or roaming data but only the travel sim.
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To be 100% sure, do make sure the "Data Roaming" is deactivated in the setting for your home line. This ensures that you can receive calls (yes, with roaming charges) but no data charges are incurring.
Depending on your carrier, you can enable Wi-Fi calling for your home line to use it abroad without additional charges. This is not the same for all carriers, do research or call support to check. In my case, if I use Wi-Fi calling with my home line I can make and take calls abroad WITHOUT any roaming charges. "Wi-Fi calling" is displayed with your carrier information if it is activated.
I hope this helps you. Don't hesitate to ask additional questions...