So the way you would be able to create a form validation on phone number that is optional is to not check the required box when setting the validtion on the form element. Create the form, and then in the server behaviors panel, you can edit the WA Server Validations and the server validation for the 2nd phone field. In the edit for this server validation check the allow blank entry checkbox. This will modify the form to allow blank entries for the server validation, but validate on phone format if there is information entered.
