A pop-up window will appear right above the system tray on every incoming phone call. You will always see at least the callers telephone number in this pop-up windows, but the more data you have about this caller, the more information will be shown. The displayable portions of the pop-up windows consist of the data fields (phone) number, first name, last name, company, address, city and country.
If you want to change the duration the pop-up remains visible, you could change its behavior in the Settings dialog.
Double-clicking the pop-up window will bring the main window back into the foreground again, if it was minimized or iconified to the system tray.