Tough one - if it's the first trunk then it may be difficult - if not put the trunks in terminal hunt for outgoing from the last to the first trunk. This way if the trunk is near the top it will minimize someone hitting it for outgoing. Like I said if it is the first trunk it could be tough due to incoming traffic. Don't know if the telco can be of any assistance in tracking the traffic. If they can then the property can note the trunk number and exact time of the call and that should be all the telco needs once you determine which trunk corresponds to telco trunks. Can't think of much else.