[WooCommerce 3.0] add phone extension field to checkout page

wc-pnef.php

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

function custom_override_checkout_fields( $fields ) {
     $fields['billing']['billing_phone_ext'] = array(
    'label'     => __('Phone ext', 'woocommerce'),
    'placeholder'   => _x('Phone ext', 'placeholder', 'woocommerce'),
    'required'  => false,
    'priority' => 105,
    'class'     => array('form-row-last'),
    'clear'     => true
     );

     return $fields;
}

add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );

function my_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('Phone ext.').':</strong> ' . get_post_meta( $order->get_id(), '_billing_phone_ext', true ) . '</p>';
}