Factory design pattern - Not use static methods because unit testing is a problem

I know this question has been asked several times in stackoverflow but somehow still had some trouble figuring out a solution. The following example I think is a good case to have static methods

public class ConnectionFactory
{
     public static Connection createConnection(ConnectionType connectionType, String ipAddr, Integer port)
    {
           //Some error checking
         switch(connectionType)
         {   
             case TCP:
                  return createTcpConnection(ipAddr, port);
             case UDP:
                  return createUdpConnection(ipAddr, port);
             case RTP:
                  return createRtpConnection(ipAddr, port);
             case SCTP:
                  return createRtpConnection(ipAddr, port);
             default:
                  break;
         }
    }

    // TcpConnection, RtpConnection, SctpConnection and UdpConnection implement interface Connection
    public Connection createTcpConnection()
    {
        Connection connection = new TcpConnection();
         .....
         .....
         return connection;
    }

    public Connection createUdpConnection()
    {
        Connection connection = new UdpConnection();
        .....
        .....
        return connection;
    }

    ....
    ....
}

And suppose if I have a CommunicationService like the following

public class CommunicationService
{
    public void initConnectionPool(ConnectionType connectionType)
    {
        for(int i = 0; i < MAX_CONNECTIONS; i++)
             connectionList.add(ConnectionFactory.createConnection(connectionType, "domain.com", 40203));

        //Some more code here to do further processing
          ......
          ......
    }    

    //Some more methods
}

Like this, different communication services can create and maintain multiple type of connections.

I want to test initConnectionPool method and in a unit test environment, socket creation will