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jQuery(document).ready(function($){
  jQuery('.test').keyup(function () {
    this.value = this.value.replace(/[^0-9]/g,'');
  });
});

<!-- http://stackoverflow.com/questions/891696/jquery-what-is-the-best-way-to-restrict-number-only-input-for-textboxes-all/11890073#11890073 -->
<input type="phone" name="quantity" class="test">