function wooc_validate_extra_register_fields( $username, $email, $validation_errors ) {

if ( isset( $_POST[‘billing_phone’] ) && empty( $_POST[‘billing_phone’] ) ) {
$validation_errors->add( ‘billing_phone_error’, __( ‘Error: Phone is required!.’, ‘woocommerce’ ) );
}
}

add_action( ‘woocommerce_register_post’, ‘wooc_validate_extra_phone_field’, 10, 3 );