code: public void addSipServletResponse(SipServletResponseImpl sipServletResponse) {
  		if(sipServletResponses == null) {
  			sipServletResponses = new CopyOnWriteArraySet<SipServletResponseImpl>();
  		}
  		sipServletResponses.add(sipServletResponse);
  	}
docs: used to get access from the B2BUA to pending messages on the transaction