I currently have voicepulse.com for their 4 trunk special. You wold think some smart guy at voicepulse.com would setup a system to monitor their usage of the customers channels. If 4 channels are being full used all the time perhaps call the client and suggest they upgrade. Well no suych luck.
My suggestion to freepbx. Along with minutes used break it down for minutes used per trunk. Extension is already available as a destination is already available although it doesnt have a nice bar chart.