in the webassist/plugins/shopping_cart/pp_expresscheckout.php page, change:
<input type="hidden" name="phone" id="phone" value="<?php echo ((isset($_SESSION["eCartCheckoutForm_phone"]))?$_SESSION["eCartCheckoutForm_phone"]:"") ?>" />
to:
<input type="hidden" name="phone" id="phone" value="<?php echo ((isset($_SESSION["eCartCheckoutForm_phone"]))?$_SESSION["eCartCheckoutForm_phone"]:"") ?>" />
<input type="hidden" name="shipping_phone" id="shipping_phone" value="<?php echo ((isset($_SESSION["eCartCheckoutForm_phone"]))?$_SESSION["eCartCheckoutForm_phone"]:"") ?>" />