From the Castel CAP IP device website, you can configure the button to call contacts in Sipelia.
In the Material configuration section, go to the Buttons/Functions Keys tab.
In the Button Label field, enter a name for the button.
From the Associated function drop-down list, select Call.
Specify the IP address to call when the button is pressed. Use the format extensiontocall@sipeliaserveripaddress.