The Telefonbuch (Phonebook) shows all internal extensions of the company, the company directory, and your own personal contacts. It is for looking things up. Administrators manage new company contacts, import, and export, see Company directory.
In the phonebook, you currently cannot call a contact with a click in the web interface; the Anrufen (Call) button is grayed out. You can call colleagues through the Team list: Your own phone rings first, and the call connects once you pick up. You dial external numbers on the phone or in the app.