Update the input form element on this page like:
<input id="Phone_Number" name="Phone_Number" type="text" value="<?php echo(WA_coalesce(array(WA_getSavedFormValue("CheckoutWizard_NewFromBlank_Default","Phone_Number"),(isset($_GET["invalid"])?ValidatedField("confirmcart","Phone_Number"):"")))); ?>" class="formTextfield_Large" tabindex="5" />
Then on the confirm_cart.php page on the top of the page, remove the server validation line that refers to the phone number field.