I  am integrating Kamailio into my application. I want to hook to the successful REGISTER and unregister event into Kamailio into my application. For now, I am able to hook into INVITE event and can hit my server to send a email to the callee user that user X is calling you. The way I am doing is by this

        if (is_method("INVITE")) {
                xlog("LOG_LOCAL3","L_INFO","invite came ($fU) ($tU)");
                $var(res) = http_connect("sipnodejsserver", "/","text/plain","src_user:$fU,dst_user:$tU" ,"$avp(route)");
                xlog("LOG_LOCAL3","L_INFO","request sent $avp(route)  $var(res)");
                setflag(FLT_ACC); # do accounting
        }

I need to show into my application that their SIP phone is online or not. How can I hook into this?