Call volume statistics

Does anyone know of an existing tool for viewing call volume statistics? More specifically, I’m trying to see a history of how many concurrent calls we have on outside lines. I’m imagining a line graph that covers whatever time period we’re interested in. Another possible solution would be seeing how many calls/minutes occur with each number of concurrent calls; it could look something like this:
1 concurrent: 100 calls, 355 minutes
2 concurrent: 30 calls, 65 minutes
3 concurrent: 4 calls, 9 minutes
4 concurrent: 1 call, 1 minute

What I’m trying to accomplish here is finding out how many lines we ought to have. We currently have 3 landlines and then a VoIP account that allows unlimited channels. However, we’re limited on how much load we’re ready to handle on the VoIP account, so it’s not as simple as just ditching all but one landline. If I knew that we rarely/never had 3 concurrent calls, it’d be very easy to decide to eliminate one of our landlines.

If this isn’t available, does anyone think this sounds like something easy to create? I’m willing to put some work into it, but I don’t want to reinvent the wheel. The “Watch Calls Graph” shows the call load I’m interested in, but only for one selected hour at a time.