Catch blank phone field

product-notify-me.liquid

{% if settings.notify_me_form %}  
  <div class="notify_form" id="notify-form-{{ product.id }}" {% if product.available %}style="display:none"{% endif %}>
    <form action="/contact" method="post" data-first-variant="{{product.variants.first.id}}" id="BIS_form" onsubmit="return createNotification{{product.id}}(this); return false;">
        <p>
        <label for="contact[email]"><strong>Contact me with stock price and availability:</strong></label>
 	</p>  
 	<p>
          <input required type="phone" name="contact[email]" id="contact[email]" placeholder="EMAIL" value="{{ contact.fields.email }}" style="display:inline;margin-bottom:0px" />
        </p>
	<p>
            <input required type="phone" name="metadata[phone]" id="contact[phone]" placeholder="PHONE" value="{{ contact.fields.email }}" style="display:inline;margin-bottom:0px" />
	</p>
	<p>
          <input class="submit" type="submit" value="Send" style="margin-bottom:0px" />    
        </p>
      <p class="BIS_response"></p>
            
    </form>
  </div>
  <script>
    var createNotification{{product.id}} = function(form) {
      if ($('[name="metadata[phone]"]').val() == '') { 
        alert('Hey, you forgot to enter a phone number!');
        return false;
      }
      
      if (BISMetadataForm) {
        var bis = new BISMetadataForm(form);
      }

      return false;
    };

    var clearBISResponse = function() { 
      $('#BIS_form .BIS_response').text('');
    };

    $(function() { 
      $('#BIS_form input').on('focus change', clearBISResponse);
      $('.product_form select').on('change', clearBISResponse);
    });

  </script>
{% endif %}