I just downloaded and this is an awesome app. My plans were to have my parents use this to communicate when they go to Europe. Works amazing on first tries in airplane mode with only wifi. So I restarted phone to see if you needed app open to contact someone or if it opened itself or what.
Here's the problem. I have the samsung moment and in the phone settings - about phone - phone number, it shows "unavailable" until the phone reaches the cell network. So even though this will work in another country in airplane mode on only wifi. It will fail to work because the phone doesn't store the phone number in the phone all the time, and the app needs access to that number to work correctly.
I posted my question on the facebook page, but I'm coming here because there's a lot of smart people as well. I don't know if other phones store the number in a similar way, but if they do this would be a huge upside to the app that doesn't quite work.