To change magento checkout validation, you can change the class of the form field to use one of the other validation methods present in js/prototype/validation.js. Just open that file and scroll down to see the methods available. Search fo r this Validation.addAllThese and you’ll see the methods below. Then open up /app/design/frontend/default/YOUR THEME/template/checkout/onepage/billing.html then change the class. For example if the class is class=”required-entry input-text” change to class=”required-entry validate-phoneLax”. This will now change the validation from simply checking if a value is present to actually checking the format.