The reason why it is displaying the page instead of a JSON response is due to the sample application's routes.

  namespace :api, path: '/', constraints: { subdomain: 'api' } do
    resources :users
  end
  constraints subdomain: ['', 'www'] do
    resources :users do 
      resources :phones
    end
    root 'users#index'
  end

In the example application's routes, there is a constraints on the subdomain where if it is empty, it will default to the web view of the application. If you're using an API only, you can remove the constraints block as well as the constraints on the subdomain: 'api'. So your routes may look something like

Rails.application.routes.draw do
  post 'user_token' => 'user_token#create'
  devise_for :users
  namespace :api, path: '/' do
    resources :users do 
      resources :phones
    end
  end
end