If you do not want to export ALL the calls on your account, you can use filters (outlined in red in the image above) to find and export the ones you want. Call records can be filtered by date, type (inbound or outbound), and/or call flags (answered, busy, fowarded, missed, sent to VM).
Click the filter buttons to set your criteria. If necessary, click "Reset Filter" to remove any parameters and display ALL calls again. Once your parameters are set, you can click the download CSV option on the right.