공짜 아니에요. 그냥 cell coverage 나쁠때 쓰라고 하는거.
밑에는 att wifi-calling faq에서 해당질문에 대한 답변
What if I dial an international number while using Wi-Fi Calling?
Calls to international numbers are charged based on the following: (1) If you have an international long distance package and you call a country in that rate plan, those rates will apply. (2) If your domestic voice plan has a reduced international calling rate and you call a country covered by your plan, those rates will apply. Otherwise, you will pay the pay-per-use rates for international long distance.
A call to a number in England will be charged at the same rate under your plan no matter where the Wi-Fi network is. It does not matter if the Wi-Fi network is in the U.S., in England, or in some other country.