code: public void addTransaction(SipServletRequestImpl request) {
          if(!request.getMethod().equalsIgnoreCase("ACK") && !request.getMethod().equalsIgnoreCase("PRACK")) {
                  String branch = ((Via)request.getMessage().getHeader(Via.NAME)).getBranch();
                  if(this.ongoingTransactions.add(new TransactionRequest(branch, request))) {
                          request.getTransactionApplicationData().setProxyBranch(this);
                          if(logger.isDebugEnabled()) {
                                  logger.debug("Added transaction "+branch+" to proxy branch.");
                          }
                  }                                               
          }                               
      }
docs: https://code.google.com/p/sipservlets/issues/detail?id=238