code: public boolean respondToCancel(SipTransaction siptrans, int statusCode, String reasonPhrase,
        int expires) {
      return respondToCancel(siptrans, statusCode, reasonPhrase, expires, null, null, null);
    }
docs: This method sends a basic response to a previously received CANCEL request. The response is
  constructed based on the parameters passed in. Call this method after waitForCancel() returns
  non-null. Call this method multiple times to send multiple responses to the received CANCEL.
  
  @param siptrans This is the object that was returned by method waitForCancel(). It identifies a
  specific CANCEL transaction.
  @param statusCode The status code of the response to send (may use SipResponse constants).
  @param reasonPhrase If not null, the reason phrase to send.
  @param expires If not -1, an expiration time is added to the response. This parameter indicates
  the duration the message is valid, in seconds.
  @return true if the response was successfully sent, false otherwise.