Wifi Calling overseas

I live overseas and kept my Verizon line active (unlimited talk/text), but only use it when connected to wifi at home, and mostly for text messages, rarely do I make or accept voice calls. I did not sign up for any international plan/services. Last night I had someone from the US call me. I had data roaming turned OFF, Wifi calling turned ON and was most definitely connected to my home wifi. I accepted the inbound call from the US and talked for an hour or so, under the impression wifi calling was free. After the call, I received a text message from Verizon because they "noticed significant usage at Pay as you Go International rates". What did I do wrong?? Did I miss a step somewhere? Do I not understand wifi calling? Thank you for any advice so I don't make this same mistake in the future.

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