This happened to me a long time ago but i totally forgot the reason.

I using a simple dispatcher, and when relaying out to the destination, kamailio is adding an additional :5060 port to the To header:

The Destination is actually 172.16.6.1:5080, i don't know why Kamailio is adding that :5060

The dispatcher is 

    if (!ds_select_dst($var(dispatcher_outbound), "4")) {
        xlog("[DISPATCH]: ds_select_dst FAILED!\n");
        send_reply("404", "No destination");
        exit;
    } else {
        xlog("[DISPATCH]: ds_select_dst was succesful\n");
    }
    xlog("L_DBG", "[DISPATCH]: going to <$ru> via <$du>\n");
    t_on_failure("RTF_DISPATCH");
    route(RELAY);

Any ideas? I totally forgot... and it's late, i guess...