I have tested this PR. Looks as when `registrar` response returns an error then Kamailio properly handles this response.
@sergey-safarov I'm sorry I don't know what you mean by this comment, do you agree that this fix is required? I'm using the 2600hz kamailio configuration to do the validation https://github.com/2600hz/kazoo-configs-kamailio/blob/4.3/kamailio/registrar...