I can suggest you to put all phonejs sources to /public folder in your rails application.
For example, I created 'assets' folder into /public and put there my phonejs application.
Next, I made simple home_controller.rb which can return some JSON data.
For retrieving data I was using the following code inside /public/assets/phonejs/views/home.js file:
var data = ko.observableArray();
$.getJSON("/home/index", function(response){
data(response);
});
var viewModel = {
data: data
};
return viewModel;
You can see phonejs app at this URL: localhost:3000/assets/phonejs/index.html
Also, you can try to put phonejs sources to vendor/assets or lib/assets or app/assets folder.
Source.zip - source code of my app (ruby 2.0, rails 4).