As long as both devices are in range of, and using wifi signals, they can communicate with each other over the internet. Therefor, there are no charges associated with these calls. Calls can be between the US and other countries, and it does not matter who originates the call -- also it certainly can be made between two travelers outside the US. However, there are a few caveats: The date and time zone settings for each of the devices MUST be set to the correct time zone that they are operating in. If not, Facetime will not work. ALSO, there are certain countries (generally in the middle east) where Facetime is blocked. If either Facetime user is in one of the blocked countries, Facetime will not work.
Again, as long as each device is using WiFi, your communications take place over the internet, so there are NO telephone company fees involved.
You're very welcome, and thank you for the feedback.
Yes, that time zone thing is critical. Most Facetime users don't have a problem with that since they probably have their location setting on Automatic. However, every so often, there is a person who reports a problem with Facetime, and the problem turns out to be date and time related.