Call charging accuracy

Why is it that I some times get charged for a call when no one answered.

It is basically because MonTel is not told if the call was answered or not by the PABX. In turn the PABX is not told by the Exchanged. MonTel can only make estimates. ALL SYSTEMS HAVE THIS PROBLEM! To find out more check out this topic: "Some important notes about call costing".

I just made a call that MonTel say was zero duration, when I actually talked for about 20 seconds or so.

It is basically because MonTel is not told the exact duration of the call by the PABX. It was told the duration of the call less about 24 seconds of potential ring time. Zero duration equal zero cost in most cases. To find out more check out this topic: "Some important notes about call costing".