Whatsapp is blocked by the Great Firewall, so it's not a great solution for China. It'd work great for Europe, though.

If you need to call real phone numbers (vs. communcating via app), Google Voice is pretty cheap: https://voice.google.com/rates (1c/min to China, 3c to HK, 1c to French landlines, 6c to French mobiles, etc.) and you can call either via VoIP in Hangouts or through the Google Voice app over the American voice network. You can't SMS international numbers at all, though, and it's a one-way street: your GV number is an American number, so while you can call the world for cheap, people calling you pay the same as calling any other U.S. number.