The problem appears to be that the fax number is actually required in that code. When I fill out all fields, it appears to validate properly.
I think you can fix that by updating the line:
$WAFV_Errors .= WAValidatePN((isset($_POST["fieldset_group_Alternate_Phone"])?$_POST["fieldset_group_Alternate_Phone"]:"") . "",false,true,true,5);
to:
$WAFV_Errors .= WAValidatePN((isset($_POST["fieldset_group_Alternate_Phone"])?$_POST["fieldset_group_Alternate_Phone"]:"") . "",false,true,false,5);