I believe the LEAF field is useful, but not too useful. Here are my points: *) It helps in unauthorized taps. I would presume that the police won't have access to the phone company's calling records if they're just using a pair of alligator clips. *) On the other hand, the system really isn't anywhere near as useful as the phone number of the person calling. There will be no map between LEAF id numbers and people. Such a map would quickly get out of date as people traded phones etc... *) It might be slightly better than the phone number in strange cases because it identifies the handset not the number. Who knows? Phone calls from the garage extension mean one thing but phones from the kitchen extension mean another. This might be significantly more important if businesses private exchanges don't release the internal extension making the call.