[Freeswitch-users] How to tell if 100 Trying received

Hi, I am trying to use FS to make outgoing SIP calls. I have a number of gateways that can make the call. However, if one of them is down or has some other problem then I would like to detect that quickly. I intended to use the provisional '100 Trying' message for this... if it hasn't been received in a couple of seconds then go on and try the next gateway. But I can't find a flag/event/state which corresponds to receipt of this message. Can anyone tell me where I should be looking? I put a debug print in sofia_event_callback for every event but there doesn't seem to be one fired for this condition. Thanks in advance. {P^/ John