@toad78,
Yes, you can add PHONE and other merge tags using a bit of custom code to hook into an action filter our plugin exposes prior to sending to Mailchimp.
add_filter( 'ss_wc_mailchimp_subscribe_merge_tags', function( $merge_tags, $order_id, $email ) {
$order = wc_get_order( $order_id );
$order_id = $order->get_id(); // Get the order ID
$merge_tags['PHONE'] = $order->get_billing_phone();
return $merge_tags;
});
Drop that into functions.php and adjust as necessary and that should send the PHONE field over to Mailchimp.