Get meta data after payment in woocommerce

I am using this to create a new field in my woocommerce billing form

add_filter('woocommerce_checkout_fields', array(__CLASS__,'custom_woocommerce_billing_fields'));
function custom_woocommerce_billing_fields($fields)
{
    $fields['billing']['billing_mobile'] = array(
        'label' => __('Mobile Phone Number', 'woocommerce'), // Add custom field label
        'placeholder' => _x('Mobile Phone Number', 'placeholder', 'woocommerce'), // Add custom field placeholder
        'required' => true, // if field is required or not
        'clear' => false, // add clear or not
        'type' => 'text', // add field type
        'class' => array('my-css')   // add class name
    );

and also i use this code to do action after payment:

add_action( 'woocommerce_payment_complete', array(__CLASS__,'create_invoice_for_wc_order'));
function create_invoice_for_wc_order( $order_id ) { 
    // get order details data...
    $order = wc_get_order( $order_id );

}

the question is how can i get that mobile number which user enter before payment in my "create_invoice_for_wc_order" function?