gistfile1.txt

$(document).ready(function() {
  var phoneLabel = $('label[for="subscription_customer_attributes_phone"]');
  phoneLabel.text(phoneLabel.text() + " *");
});

var form = $("form:first");
var phone = $("#subscription_customer_attributes_phone");
var submitBtn = $("#subscription_submit")

submitBtn.click(function(){
  if (phone.val() === "") {
    phone.addClass("field-error");
    return false;
  }
});

form.change(function() {
  if (phone.val() != "") {
    submitBtn.click(function() {
      return true;
    });
  }
});

$("#subscription_customer_attributes_phone").blur(function() {
  if ($(this).val() !== "") {
    $(this).removeClass("field-error");
  } else {
    $(this).addClass("field-error");
  }
});

.field-error {
  border-color: #ff0000;
}