Default phone styling is a bit of a pain, what i ended up doing is just using CSS to style that section. It looks fine if you have Kentico's core css class, but i would rather add a small rule and be done than try to resolve any styling breaking by introducing Kentico's core style.
Otherwise you can set the form field to a textbox instead, if you add the condition "Is USA phone number" in the validation it still catches things well.