Kamailio

# Basic Kamailio routing script
request_route {
    # per request initial checks
    route(REQINIT);
    
    # handle requests within SIP dialogs
    route(WITHINDLG);
    
    # handle registrations
    route(REGISTRAR);
    
    # handle routing
    if (!route(RELAY)) {
        sl_send_reply("500", "Server Error");
    }
}

route[REQINIT] {
    if (!mf_process_maxfwd_header("10")) {
        sl_send_reply("483", "Too Many Hops");
        exit;
    }
    
    if (!sanity_check("1511", "7")) {
        xlog("Malformed SIP message from $si:$sp\n");
        exit;
    }
}

route[WITHINDLG] {
    if (has_totag()) {
        if (loose_route()) {
            route(RELAY);
        } else {
            if (is_method("ACK")) {
                if (t_check_trans()) {
                    route(RELAY);
                    exit;
                }
            }
            sl_send_reply("404", "Not here");
        }
        exit;
    }
}

route[REGISTRAR] {
    if (is_method("REGISTER")) {
        if (!save("location")) {
            sl_reply_error();
        }
        exit;
    }
}

route[RELAY] {
    if (!t_relay()) {
        sl_reply_error();
    }
    exit;
}