Out of the box the RGS reports are quite good. The biggest complaint I get is when customers see the abandoned calls percentage then ask "Why were these abandoned?".
You have to drill down into offered calls and work it out on the SIP response codes. It would be a lot better if there was an "Unanswered Calls" column that you could drill down into. This would just need to show the call and why it wasn't answered. E.g. Bob pressed cancel and Bill didn't pick up. That's what people want to know.