I own a startup and my work often takes me to India, Philippines and Japan and I try to use WiFi to make calls to my family back home. I’m using Viber to make these voip calls but after a latest update calls get disconnected and on few occasions I’ve been overcharged. Since my visits are short term no point in buying a budget SIM overseas. I’m looking for alternatives.
Most popular is Skype! It works well from any countries. If it's to expensive for you, you may didn't have a choice for to use services with less power, all over avaibility and more! It's also may be not work in any countries or have some (other) limitations. You'll have no choice for to check out those service providers totally on your own. Sorry for a not helpful post!