Unfortunately Whistle doesn't work in areas with poor coverage. And then there's the battery issues. A tag would at least be a good redundancy solution.
We live the mountains with poor coverage and a dog who at one time was a wizard at escaping and getting lost.
Yet we do have people all around, so I think the iphone solution would work, even if it was delayed. At least I would have an idea of where to start searching. I bought every lower than professional level device and they all just didn't work here.