It depends on the signalling protocol you are using. If you are using MGCP on your 3640, then you can use performance monitor on your CM to view your active channels throughout the day. If you are using H.323 (more likely), then I would look at something like CWSI to help you here. I'm not even 100% sure that you could get what your're looking for, but the new voice health monitor might have this capability. Also, take a look at ART to help generate call reports for you.