If you have CSS Form Builder or the older Validation Toolkit you can select the element and update the client validation that is applied to it. There are a couple of reformatting options available.
You can also try just updating the mask that is used for the element. On the page you can search for 'x (xxx) xxx-xxxx', there will be one occurrence of this for the phone and one for the fax. You should try updating the mask to fit the format that you need here like this: