dispatcher module and rewritehost vs forward

 I have the following in my config, which works...
now if I comment forward and comment out rewritehost I get errors. Am I not using the correct syntax? 

route[5] {

        # -----------------------------------------------------------------
        # PSTN Handler
        # -----------------------------------------------------------------

        t_on_reply("1");

        # do load balancing
        ds_select_dst("1","4");
        sl_send_reply("100","Trying");

        #rewritehost(uri:host);
        forward(uri:host,uri:port);

        route(4);
        route(1);
}