add_action( 'profile_update', 'custom_update_checkout_fields' ); // Fires immediately after an existing user is updated.
add_action( 'user_register', 'custom_update_checkout_fields' ); // Fires immediately after a new user is registered.
function custom_update_checkout_fields( $user_id ) {
$current_user = get_userdata( $user_id ); #Here we use get_userdata() and not wp_get_current_user().
#Updating info
if ( $current_user->user_firstname != $current_user->billing_first_name )
update_user_meta( $user_id, 'billing_first_name', $current_user->user_firstname );
if ( $current_user->digits_phone != $current_user->_rtcl_phone )
update_user_meta( $user_id, '_rtcl_phone', $current_user->digits_phone );
if ( $current_user->digits_phone != $current_user->_rtcl_whatsapp_number )
update_user_meta( $user_id, '_rtcl_whatsapp_number', $current_user->digits_phone );
}