That is a great idea. It must be some kind of thermoelectric junction stack, the water cools one side. The more temp differential, the more efficient they are. Sort of like the millivolt stack in my furnace.
It would be much more efficient to burn the fuel in a little engine, but this is simple and can be put over any flame source. But I bet it is still pretty inefficient in terms of having to burn a whole tank of propane to talk on the phone, that is probably several kilowatts-hours of energy in to get a couple of watt hours out.