code: public SipServletRequest getLinkedSipServletRequest(SipServletRequest req) {
          if (req == null) {
              throw new NullPointerException("the argument is null");
          }
          return linkedRequestMap.get(req);
      }
docs: /*
  (non-Javadoc)
  @see javax.servlet.sip.B2buaHelper#getLinkedSipServletRequest(javax.servlet.sip.SipServletRequest)