This is pure speculation, but I would think you could use the Open Source SMDR app that Ralph found
http://simplesmdr.sourceforge.net/. Now I don't think the 2000SX does IP SMDR so I think you would need to convert the serial data coming from the PBX to IP. I don't know how much SMDR has changed from the 2000sx to the 3300 - IE: if it will be compatible.
I use a Qualtech ThinkQ QSE-100D that I bought for $25 on ebay for some serial applications I have. (They seem to be in the $350 range on eBay right now). The trick here would be confirming that if this box is running in RAW mode the data would be in a format that Simple SMDR could read.
This one is a long shot, and would be some work, but might fit your need. The hard part here is if you get stuck there is really nobody to call for help....and it's just a theory of mine that it would work.
Now, if you spend some more, I've had good luck with MicroCall Accounting package:
http://www.microcall.com/ Good luck!
-Chak