What is the best app for calling back to the U.S. when travelling abroad?

Using your data connection is easiest, Hangouts, Skype, etc. Hangouts uses your Google Voice number if you have that set up. If you need cellular, then that's not going to be an app - just need service from a carrier in the country that you're in.
 
I've used Skype quite a bit (low cost, buy calling credit as you go, and you can buy a Skype number for a one time fee). If you use Google Voice, you can buy cheap calling credits as well. Carriers like T-Mobile offer free WiFi calling to US (discounted non-WiFi calls as well as free text messaging).
 

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