in your form try this:
<%= f.input :phone, :required => true,
:input_html => { :maxlength => 14, :"data-mask" => "999-999-9999" } %>
Also, assuming you're running Rails 3.1+ and you have the jasny files in your vendor/assets/javascripts directory, be sure to include the bootstrap-inputmask file in app/assets/javascripts/application.js:
//= require jquery
//= require jquery_ujs
//= require bootstrap
//= require bootstrap-inputmask
//= require_tree .