Generally speaking, all information regarding Call Center calls is stored in Actions for the solicitor (caller) or the constituent being called. Reports in the Call Center may change or not be available depending on the version of Call Center in use.

Otherwise, the Call Center doesn’t really have a concept for how long any one specific call took. Likewise, the actions stored on the called constituent do not populate a start and end time. The hours seen on detail reports are derived from the total time on the solicitor’s action (difference between start time and end time on the solicitor action). The number of calls/hour is the total calls divided by the number of hours.  

It is worth pointing out that each constituent that is called should also see an action on their constituent record listing the person calling as a solicitor. This means it should be possible to come up with the same numbers as the report by performing a query in RE on constituent Action with our solicitor between a time frame to come up with the number of calls performed.

If needed, please see the following article regarding reports that are not totaling correctly in the Call Center:
Unable to view Caller reports or the Call Center reports are inaccurate - when using the Call Center Add-in