Nothing. It just works. As long as the telco is sending it, it will be displayed on the screen of the digital phones. If you're using analog phones, make sure they're compatable in your country with the CLI standards (not some cheap knock offs you've picked up somewhere).
If you're getting WITHHELD on the screen of the phone then the caller has withheld their number and you can't force it to be disalayed.
What type of lines to you have - BRI/PRI/analog?