code: public void sendCallMessage(SignalServiceAddress recipient,
                                Optional<UnidentifiedAccessPair> unidentifiedAccess,
                                SignalServiceCallMessage message)
        throws IOException, UntrustedIdentityException
    {
      byte[] content = createCallContent(message);
      sendMessage(recipient, getTargetUnidentifiedAccess(unidentifiedAccess), System.currentTimeMillis(), content, false);
    }
docs: Send a call setup message to a single recipient.
  
  @param recipient The message's destination.
  @param message The call message.
  @throws IOException