I am looking for basic Call Accouting for Cisco Unified Communications 520
Here is one Cisco Compatible Call Accounting Product for the UC520 from "Stonevoice"
Billy2.0 for UCME - UC500
Hope this helps!
If you upgrade to UC 4.2(7), that will give you CUE 3.0
That will enable you to use the "Client Historical reporting", see:
Hope this helps, please rate post if it does!