Setup the function keys (speed dial) for fast calling.

If you want to configure the white buttons adjacent to the phone screen, open https://isb2.wildixin.com/collaboration/ login and go to settings. Select the Fn keys menu from the left panel.

You can remove Line 1 and 2 - the default setup - and add a Colleague. 

Settings

Change the type of function that the button should have to Colleague, add an appropriate name (it will be visible on the phone), and search by name or input the extension. Do not forget to click on the eye to stop the notifications from popping up when your colleague is on a call and press Save at the end.