JQuery :: Validate Optional Fields Only When Data Is Entered?

I'm using jQuery validation on a form and I can't get it to only validate optional fields if the user enters something (should recognize the difference between this and the default value of the field, for text input fields). I know this is built in (I think) but I must be doing something wrong.

For example, phone number is optional but I want it to validate if the user enters something. Code for the phone in the JS file is:

jQuery.validator.addMethod("phone", function(phone_number, element) {
phone_number = phone_number.replace(/s+/g, "");
return this.optional(element) || phone_number.length > 9 &&
phone_number.match(/^(1-?)?(([2-9]d{2})|[2-9]d{2})-?[2-9]d{2}-?d{4}$/);

[Code]....

When I just put "phone" as the class it validates the field even if it is not edited by the user at all. For the required fields I put "required" as the class to validate them but I just read in another post that this is not necessary with the plugin...? I guess I am not sure whether the field should be define as optional by a class or in the JS file, and if it's the latter, how to do that.