WooCommerce - Validating new "My account" registration fields

functions.php

<?php
/**
 * Validate the extra register fields.
 *
 * @param WP_Error $validation_errors Errors.
 * @param string   $username          Current username.
 * @param string   $email             Current email.
 *
 * @return WP_Error
 */
function wooc_validate_extra_register_fields( $errors, $username, $email ) {
    if ( isset( $_POST['billing_first_name'] ) && empty( $_POST['billing_first_name'] ) ) {
        $errors->add( 'billing_first_name_error', __( '<strong>Error</strong>: First name is required!', 'woocommerce' ) );
    }

    if ( isset( $_POST['billing_last_name'] ) && empty( $_POST['billing_last_name'] ) ) {
        $errors->add( 'billing_last_name_error', __( '<strong>Error</strong>: Last name is required!.', 'woocommerce' ) );
    }


    if ( isset( $_POST['billing_phone'] ) && empty( $_POST['billing_phone'] ) ) {
        $errors->add( 'billing_phone_error', __( '<strong>Error</strong>: Phone is required!.', 'woocommerce' ) );
    }

    return $errors;
}

add_filter( 'woocommerce_registration_errors', 'wooc_validate_extra_register_fields', 10, 3 );