As a long time business traveler can I just share one recommendation for low cost calling.
Look up TALK360, this Netherlands based service gives great calling rates to and from anywhere in the world.
I use it to make all my calls to clients as it's by the far the most cost effective service I have found.
As long as you have a decent internet connection the call quality is superior to any other service I have tried. (I've tried them all!).
Very easy to use with a good app.
Check it out for yourself.