If you set the 'line' parameter the call will fail (I suppose you don't have a line '100' registered to another server).
NOTE: If you want to call on a different path a busy target you should be aware you might call again to someone that rejected the call!
If you want to fallback to another target use fork.
See 
http://docs.yate.ro/wiki/Call_ForkerThe handler should look like this:
if ($ev->getValue("called")=="100") {
  $ev->retval = "fork";
  $ev->params["callto.1"] = $location;
  $ev->params["callto.2"] = "|";
  $ev->params["callto.3"] = PUT_HERE_THE_SECOND_TARGET;
  $ev->params["callto.1.maxcall"] = PUT_HERE_MAX_CALLING_TIME_IN_MILISECONDS;
  $ev->params["stoperror"] = "noanswer|busy^";
  $ev->params[""] = ;
  $ev->handled = true;
  $ev->Acknowledge();
  break;
}