add_filter( 'woocommerce_sagepay_direct_data', 'telephone_order_payment', 10, 2 );

gistfile1.txt

add_filter( 'woocommerce_sagepay_direct_data', 'telephone_order_payment', 10, 2 );
function telephone_order_payment( $data, $order ) {

  $order_id = $order->get_id();

  $type = get_post_meta( $order_id, '_created_via', TRUE);

  if ( $type !== 'checkout' ) {
    $data['AccountType'] = 'M';
  }

  return $data;
  
}