https://support.google.com/hangouts/answer/6079055?p=android_first_minute&rd=1
Almost all calls to the US and Canada are free from all countries where
Hangouts calling is available. A few US and Canadian destinations will cost
USD 0.01 per minute depending on the specific phone number.
刚测试过APP 拨打电话，超过一分钟，没计费。
但用gmail hangout拨打相同号码则计费。
这东西太猛了...竟然可以做到跨国(美加）打多数电话免费。