By clicking on Unknown in the title bar you change into the Unknown Numbers mode of operation. In contrast to the main phone book view, only those numbers are displayed in the phone book and CDR lists, that are not known to the the phone book. This list is intended mainly for database maintenance purposes. You can name any number by clicking on it, adding a descriptive name into the Name field, and pushing the green Store-Button (+). As soon as this number is stored, it will immediately disappear from the Unknown Number lists.
By clicking on the Call Count column you can reverse the sorting order so that the most often called numbers are listed first, which is handy if you want to name only the most popular telephone numbers.
Please note that the phone book and CDR lists only show a predefined maximum number of lines in order to get a good interactive performance and limit the CPU burden on the Asterisk server. Your administrator can define the number of lines by setting the max_result parameter. It depends on the filtering criteria and on the daily number of phone calls how far the calling hisory actually reaches back.
If you see an s as the destination address in the CDR menu, this is the so called Asterisk Standard Extension. It indicates that Asterisk was unable to keep track of which extension has actually answered the call and fell back to a dial plan effort of last resort. This is not necessarily a malfunction, but your Asterisk administrator should be able to improve this behaviour.
Note: Calls with the internal tag invalid are not displayed in the CDR list, as well as dummy calls to your own extension with zero duration.