Or you could try something like FireChat to make a mesh network that doesn't even require a data or WiFi connection. (Not sure there are enough users in Hanoi to facilitate this though.)
That's what I was thinking. Apps like Roger are pretty much useless to me. If I have cell coverage or wifi, I don't really need a walkie-talkie app. Most places I am, I have neither. I also don't have enough user density for a mesh network. So I think my walkie-talkie app will be a walkie-talkie and, for near range comm, my exceptionally loud voice, for the foreseeable future.
