When calling some numbers I have to forward calls to a router cisco (then it wil forward calls out through isdn) or to a backup router cisco if the first one is busy. The route to primary cisco router works but I don't know how to implement the backup route in ser.cfg ? how can I configure a backup route in sip server?

if ((uri=~"^sip:80[0-9]+@") | (uri=~"^sip:1[0-9][0-9]@") | (uri=~"^sip:1515@")) {
                rewritehost("router IP");
                forward(uri:host, uri:port);
                break;
        };