<?php
// Do NOT include the opening php tag.
// Place in your theme's functions.php file

/**
 * Add a custom field (in an order) to the emails
 */
add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );

function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['meta_key'] = array(
        'label' => __( 'Shipping Phone' ),
        'value' => get_post_meta( $order->id, '_shipping_phone', true ),
    );
    return $fields;
}