Great question! A call status of "Error" denotes trouble connecting the call. A call with an "Error" status has never been connected. A call may have an error status due to the following reasons:
1) The Local telephone carrier is temporarily overloaded. (This would be the equivalent of you calling a friend and hearing "All circuits are busy at this time. Please try again later.)
2) A high dialing speed to a certain location can cause the telecom provider to reject calls. For example, calling at 50 calls per minute (cpm) to a small town, or suburb, may be overwhelming for the telephone company in that area. A good way to fix this: Mix lists containing numbers from different cities & states. You can also reduce the speed of your campaign.
3) The phone number is temporarily, or permanently disconnected.