If you want to download a list of your calls for any given period of time, you need to login to your account and choose Messages options on the left pane.
From the list of messages click on the box at the top left of messages to select them all or select particular messages by clicking the boxes next to the individual messages.
Then, click on the options menu (the three dots at the top right corner of the list of messages). You’ll see an option to print your call details. From the ensuing screen you can choose to save the call details as a PDF.