code: public void removeTransaction(String txId) {
              if(this.transactionMap.remove(txId) != null) {
                      if(logger.isDebugEnabled()) {
                              logger.debug("Transaction "+txId+" removed from proxy.");
                      }
              }
              checkAndCleanProxy();
      }
docs: https://code.google.com/p/sipservlets/issues/detail?id=238