I've been looking at this for our new environment as our sales team like the 'ring all' strategy for incoming calls. We also overflow from one group (with a small number of users) to a larger group if the call is not answered in the first couple of rings.
If you run call stats on the groups, then you see a missed call from the first group (even if it was answered in the second group) which is correct if you are reviewing the performance of the first group, but incorrect if you are reviewing missed customer calls....
There is a report that shows missed calls by the incoming DDI number - that shows the call as answered (regardless of which group answers it) - which means we can accurately show the 'customer missed calls'.
Maybe this will work for you also?