Another useful monitoring tool is Performance Monitor. If you open Performance and add a counter, you will see 3CX PerfMon listed on the drop-down menu. There you will see some great monitoring options. The number of calls completed (active, successful, or failed), and totals of other items are all worth looking at on a regular basis. Turning these on will take up some CPU time, so enable only the ones you need or check it often to keep track of how your system is doing.
Performance Monitoring unfortunately does not appear to be a high priority and has stopped working in some versions. Before you spend a lot of time trying to read these, make sure they work in the version you have.
See http://www.3cx.com/forums/performance-monitors-11913.html#p62702 for more information.