Call logs are created by CTI (Computer-Telephony Integration) system. See Computer Telephony Integration - CTI.
If the Ticket tab is selected in left panel this tab displays a list of call logs attached to the ticket.
In form there is a button, Attach Log, in a form you can select logs to attach. You can View logs and Detach them from the ticket. Above list there is a checkbox filter to list only logs containing comment (With comment) and a dropdown filter to select log source type (sources are different telephone systems, eg Telavox, Skype, Lync).
If User tab is selected in left panel, the tab displays a list of logs where the user is defined as Caller. In this case there is an additional checkbox filter to show only logs that are connected to a ticket and a button to create a new log (Add log).
This tab is not shown if Organization is selected in left panel.