I just tried that, but it looks strange.

This is how my route for t_relay looks like:

route[2] {

        xlog("L_DBG", "Sending out packet $rm stateful.\n");
        if (is_method("INVITE")) {
                xlog("L_INFO", "REQUEST: Open branches: $avp(s:branches) T=$tU\n");
                if (defined ($avp(s:branches))) {
                        xlog("L_INFO", "REQUEST: branches avp is defined\n");
                        $avp(s:branches) = $avp(s:branches) + 1;
                        xlog("L_INFO", "REQUEST: branches avp is now: $avp(s:branches) T=$tU\n");
                } else {
                        xlog("L_INFO", "REQUEST: branches avp is NOT defined\n");
                        $avp(s:branches) = 1;
                        xlog("L_INFO", "REQUEST: branches avp is now: $avp(s:branches) T=$tU\n");
                }
        }
        if (!t_relay()) {

                xlog("L_ERR", "route(2): error to <$tu> from <$fu>\n");

                sl_reply_error();
                return;
        };
        return;
}

Additionally I have a route called from a named reply_route, where I print the number of branches:

xlog("L_INFO", "REPLY: Number of open branches: $avp(s:branches) T=$tU\n");

Now, when I run this code, calling the extension 1016908e0 with two devices registered to it, the log gives me the following output:

Jun 12 08:39:26 hagi /usr/sbin/kamailio[32497]: INFO: <script>: REQUEST: Open branches: <null> T=1016908e0
Jun 12 08:39:26 hagi /usr/sbin/kamailio[32497]: INFO: <script>: REQUEST: branches avp is NOT defined
Jun 12 08:39:26 hagi /usr/sbin/kamailio[32497]: INFO: <script>: REQUEST: branches avp is now: 1 T=1016908e0
[..]
Jun 12 08:39:31 hagi /usr/sbin/kamailio[32499]: NOTICE: <script>: reply: 486 Busy Here - M=INVITE F=sip:021163555552@sipgate.de T=sip:1016908e0@sipgate.de SRCIP=217.10.66.131:5070
Jun 12 08:39:31 hagi /usr/sbin/kamailio[32499]: INFO: <script>: REPLY: Number of open branches: 18446744073709551613 T=1016908e0

Both phones do ring. I would expect to see the REQUEST part twice, once for each branch, but it is only executed once. And the AVP doesn't survive, carries a completely wrong value.

Do you have any idea, why Kamailio is doing this?

Best Regards,
Sebastian