You can assign a status to each call so that you can keep track of those that have been completed, need follow-up, are awaiting reply etc. The list of available statuses can be changed in the preferences.

Use the status filter drop-down located above the call list to restrict the calls that are displayed to those with a particular status. Eg, you can choose to show only calls for which you need to ring people back, by choosing to filter for the 'Follow Up' status.