Hi Ian,
FormBuilder uses both server side and client side validation, so the Spry (client side) character masking (which is enabled by making sure that the 'Enforce Pattern' check box has been ticked in the spry panel) writes the option to the javascript textfield widget for the phone textfield at the bottom of the page.
So now I'm confused, what got written to the head code in your page, and how did you do it?