Pressing the little icon will pop-up the Phonebook selection dialog :

Here you can select a phonebook entry and press to connect to this station. If the Station is a client RTP destination it will connect to this station, if it is a Server profile it wil select the profile and wait for incoming connections.

If the selected station is a SIP destination, LUCI STUDIO will: disconnect previous station, register with the default SIP server and call the number.

After the call is finished LUCI STUDIO will be registered and the selected number will be kept , so you could call it again or try later when failed. If you want to revert to the previous LUCI STUDIO station, just press the button.