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.
剛測試過APP 撥打電話，超過一分鐘，沒計費。
但用gmail hangout撥打相同號碼則計費。
這東西太猛了...竟然可以做到跨國(美加）打多數電話免費。